Showing 1–50 of 133 results for author: Bouquin, J - L

Discovery of a star sensitive to the spin of Sgr A*

Authors: K. Abd El Dayem, R. Abuter, N. Aimar, P. Amaro-Seoane, A. Berdeu, J. -P. Berger, G. Bourdarot, W. Brandner, A. Burkert, D. Calderon, C. Correia, J. Cuadra, R. Davies, D. Defrere, L. Delit, A. Drescher, F. Eisenhauer, L. Esteras Otal, M. Fabricius, H. Feuchtgruber, N. M. Foerster Schreiber, A. Foschi, P. Garcia, R. Garcia Lopez, A. Generozov , et al. (63 additional authors not shown)

Abstract: Residing in the center of the Milky Way, Sgr A* is the closest massive black hole (MBH). Its vicinity has allowed measuring individual stellar orbits around it. The stars act as test particles and probe the gravitational potential around the $4.3 \times 10^6 M_\odot$ MBH. These observations have determined the central mass to sub-percent precision, and the mildly relativistic motions of stars have… ▽ More Residing in the center of the Milky Way, Sgr A* is the closest massive black hole (MBH). Its vicinity has allowed measuring individual stellar orbits around it. The stars act as test particles and probe the gravitational potential around the $4.3 \times 10^6 M_\odot$ MBH. These observations have determined the central mass to sub-percent precision, and the mildly relativistic motions of stars have given access to the dominant relativistic corrections, the gravitational redshift, the transverse Doppler effect, and the prograde precession imposed by the Schwarzschild metric nature of the potential. These effects are of order $β^2 = (v/c)^2$ (for velocity $v$ and speed of light $c$). The Kerr metric for a rotating black hole leads to corrections of order $β^3$. Here, we report the discovery of a faint main-sequence star ($m_K = 19.3$), S301, on a 8.7-year orbit and with small enough a pericenter distance, such that the star's peak velocity reaches $25000\,$km/s. Within the measurement capabilities of current near-infrared interferometry and future spectroscopy on an extremely large telescope, S301's motion is directly sensitive to the spin of Sgr A*. The high eccentricity of S301 suggests that it is the captured component of a binary that was torn apart via the Hills mechanism. △ Less

Submitted 14 July, 2026; originally announced July 2026.

Comments: accepted for publication in Nature

Southern Massive Stars at High Angular Resolution (SMaSH+): Properties of hierarchical massive triples

Authors: E. Bordier, H. Sana, A. J. Frost, A. -S. Libert, J. Vrancken, L. Mahy, S. Toonen, F. Tramper, A. de Koter, S. Lacour, J. -B. Le Bouquin, W. -J. de Wit

Abstract: While massive stars are frequently found in triple architectures, the lack of observed parameter distributions has long remained a bottleneck for statistical models of their evolution. We compile the first representative set of physical and orbital distributions for main-sequence hierarchical massive triples. We present a homogeneous analysis of 26 O-type hierarchical triples identified in the SMa… ▽ More While massive stars are frequently found in triple architectures, the lack of observed parameter distributions has long remained a bottleneck for statistical models of their evolution. We compile the first representative set of physical and orbital distributions for main-sequence hierarchical massive triples. We present a homogeneous analysis of 26 O-type hierarchical triples identified in the SMaSH+ survey by combining spectroscopic data for inner binaries with interferometric and aperture masking detections of tertiary companions within $\sim$200 au. We derive the distributions of masses, mass ratios, and separations, and investigate their joint probability density functions. We assess the dynamical stability of these systems and estimate the relative importance of secular processes by comparing the von Zeipel-Kozai-Lidov (ZKL) timescale to the general relativistic precession timescale for five systems with well-constrained orbital solutions. Finally, we evaluate the observational completeness. The sample is dominated by strongly hierarchical configurations, consisting primarily of tight inner spectroscopic binaries(a_in< 1 au) and wider tertiaries (a_out/a_in $>$ 70 for most systems). We find no significant correlation between tertiary mass and either inner-binary mass or outer separation, indicating a broad diversity of system architectures. Ten systems host relatively massive tertiaries (q_out>0.5), especially at closer outer separations (a_out$\lesssim$30 au). For two to four systems out of five, general relativistic precession dominates over ZKL oscillations in their current configuration. These results provide the first observationally grounded distributions of key parameters for massive hierarchical triples and offer important constraints for population synthesis and evolutionary models, particularly regarding the role of tertiary companions in shaping binary evolution. △ Less

Submitted 26 June, 2026; originally announced June 2026.

Comments: 18 pages including 5 pages Appendix

The GRAVITY young stellar object survey -- XV. The star-disk interaction region of the T Tauri star DO Tau

Authors: GRAVITY Collaboration, K. Perraut, J. Bouvier, H. Nowacki, A. Sousa, M. Houllé, J. F. Donati, E. Alecian, S. Alencar, M. Audard, J. -P. Berger, Y. -I. Bouarour, E. Bordier, G. Bourdarot, A. Carmona, A. Caratti o Garatti, C. Dougados, M. Flock, R. Garcia-Lopez, K. Grankin, A. Kospal, L. Labadie, C. Moutou, J. Sanchez-Bermudez, H. Shang , et al. (39 additional authors not shown)

Abstract: Protoplanetary disks around young Sun-like stars are the cradles of the vast majority of detected exoplanets. Probing these disks at multiple spatial scales is key to uncovering how planets form. We aim to spatially and spectrally resolve the inner disk and star-disk interaction region of the M0.3 T Tauri star DO Tau by combining two complementary techniques. We used high-resolution near-infrared… ▽ More Protoplanetary disks around young Sun-like stars are the cradles of the vast majority of detected exoplanets. Probing these disks at multiple spatial scales is key to uncovering how planets form. We aim to spatially and spectrally resolve the inner disk and star-disk interaction region of the M0.3 T Tauri star DO Tau by combining two complementary techniques. We used high-resolution near-infrared spectra from CFHT/SPIRou to constrain the magnetospheric star-disk interaction process and optical long-baseline interferometry with ESO VLTI/GRAVITY to determine the sizes of the K-band continuum and Br$γ$ line emitting regions. From the SPIRou spectra, we confirmed that this ~0.5 M$_\odot$ star is a strong accretor. The HI and HeI lines exhibit strong variability on a daily timescale, consistent with the burster classification of DO Tau derived from its K2 light curve. We derived an upper limit of 0.35 on the ratio between the magnetospheric truncation radius and the disk corotation radius, indicative of an ordered unstable accretion regime. The size of the Br$γ$ line emitting region obtained from GRAVITY is much smaller than the K-band continuum emitting region. This compact Br$γ$ emission region ($R_{Brγ} \sim$ 0.011 au) suggests that most of the line flux originates from the magnetospheric accretion region and/or from an inner wind close to the magnetosphere-disk interface. The inclination we derived for the inner disk (45-55°) differs from that of the outer disk inferred from the ALMA continuum (30°). This points toward a misalignment or warp of the outer disk that may originate from the suspected past encounter with the neighboring HV Tau system. △ Less

Submitted 23 March, 2026; originally announced March 2026.

Exomoon search with VLTI/GRAVITY around the substellar companion HD 206893 B

Authors: Q. Kral, J. Wang, J. Kammerer, S. Lacour, M. Malin, T. Winterhalder, B. Charnay, C. Perrot, P. Huet, R. Abuter, A. Amorim, W. O. Balmer, M. Benisty, J. -P. Berger, H. Beust, S. Blunt, A. Boccaletti, M. Bonnefoy, H. Bonnet, M. S. Bordoni, G. Bourdarot, W. Brandner, F. Cantalloube, P. Caselli, G. Chauvin , et al. (108 additional authors not shown)

Abstract: Direct astrometric detection of exomoons remains unexplored. This study presents the first application of high-precision astrometry to search for exomoons around substellar companions. We investigate whether the orbital motion of the companion HD 206893 B exhibits astrometric residuals consistent with the gravitational influence of an exomoon or binary planet. Using the VLTI/GRAVITY instrument, we… ▽ More Direct astrometric detection of exomoons remains unexplored. This study presents the first application of high-precision astrometry to search for exomoons around substellar companions. We investigate whether the orbital motion of the companion HD 206893 B exhibits astrometric residuals consistent with the gravitational influence of an exomoon or binary planet. Using the VLTI/GRAVITY instrument, we monitored the astrometric positions of HD 206893 B and c across both short (days to months) and long (yearly) timescales. This enabled us to isolate potential residual wobbles in the motion of component B attributable to an orbiting moon. Our analysis reveals tentative astrometric residuals in the HD 206893 B orbit. If interpreted as an exomoon signature, these residuals correspond to a candidate (HD 206893 B I) with an orbital period of approximately 0.76 years and a mass of $\sim$0.4 Jupiter masses. However, the origin of these residuals remains ambiguous and could be due to systematics. Complementing the astrometry, our analysis of GRAVITY $R=4000$ spectroscopy for HD 206893 B confirms a clear detection of water, but no CO is found using cross-correlation. We also find that AF Lep b, and $β$ Pic b are the best short-term candidates to look for moons with GRAVITY+. Our observations demonstrate the transformative potential of high-precision astrometry in the search for exomoons, and proves the feasibility of the technique to detect moons with masses lower than Jupiter and potentially down to less than Neptune in optimistic cases. Crucially, further high-precision astrometric observations with VLTI/GRAVITY are essential to verify the reality and nature of this signal and attempt this technique on a variety of planetary systems. △ Less

Submitted 25 November, 2025; originally announced November 2025.

Comments: accepted for publication in A&A

Journal ref: A&A 705, A217 (2026)

The ExoGRAVITY survey: A K-band spectral library of giant exoplanet and brown dwarf companions

Authors: J. Kammerer, T. O. Winterhalder, S. Lacour, T. Stolker, G. -D. Marleau, W. O. Balmer, A. F. Moore, L. Piscarreta, C. Toci, A. Mérand, M. Nowak, E. L. Rickman, L. Pueyo, N. Pourré, E. Nasedkin, J. J. Wang, G. Bourdarot, F. Eisenhauer, Th. Henning, R. Garcia Lopez, E. F. van Dishoeck, T. Forveille, J. D. Monnier, R. Abuter, A. Amorim , et al. (84 additional authors not shown)

Abstract: Direct observations of exoplanet and brown dwarf companions with near-infrared interferometry, first enabled by the dual-field mode of VLTI/GRAVITY, provide unique measurements of the objects' orbital motions and atmospheric compositions. Here, we compile a homogeneous library of all exoplanet and brown dwarf K-band spectra observed by GRAVITY thus far. We re-reduced all the available GRAVITY dual… ▽ More Direct observations of exoplanet and brown dwarf companions with near-infrared interferometry, first enabled by the dual-field mode of VLTI/GRAVITY, provide unique measurements of the objects' orbital motions and atmospheric compositions. Here, we compile a homogeneous library of all exoplanet and brown dwarf K-band spectra observed by GRAVITY thus far. We re-reduced all the available GRAVITY dual-field high-contrast data and, where companions are detected, extract their ~2.0-2.4 $μ$m K-band contrast spectra. We then derived stellar model atmospheres for all employed flux references, which we used to convert the companion contrast into companion flux spectra. Solely from the resulting GRAVITY spectra, we extracted spectral types, spectral indices, and bulk physical properties for all companions. Finally, and with the help of age constraints from the literature, we also derived isochronal masses for most companions using evolutionary models. The resulting library contains R ~ 500 GRAVITY spectra of 39 substellar companions from late M to late T spectral types, including the entire L-T transition. Throughout this transition, a shift from CO-dominated late M- and L-type dwarfs to CH4-dominated T-type dwarfs can be observed in the K-band. The GRAVITY spectra alone constrain the objects' bolometric luminosity to typically within $\pm$0.15 dex. The derived isochronal masses agree with dynamical masses from the literature where available, except for HD 4113 c for which we confirm its previously reported potential underluminosity. Medium-resolution spectroscopy of substellar companions with GRAVITY provides insight into the carbon chemistry and the cloudiness of these objects' atmospheres. It also constrains these objects' bolometric luminosities which can yield measurements of their formation entropy if combined with dynamical masses, for instance from Gaia and GRAVITY astrometry. △ Less

Submitted 16 January, 2026; v1 submitted 9 October, 2025; originally announced October 2025.

Comments: 21 pages, 11 figures, published in A&A

Journal ref: A&A 704, A318 (2025)

First Light for the GRAVITY+ Adaptive Optics: Extreme Adaptive Optics for the Very Large Telescope Interferometer

Authors: GRAVITY+ Collaboration, :, F. Allouche, C. Bailet, M. Benisty, A. Berdeu, J. -P. Berger, P. Berio, A. Bigioli, C. Blanchard, O. Boebion, H. Bonnet, G. Bourdarot, P. Bourget, W. Brandner, J. Brulé, P. Burgos, M. Carbillet, C. Correia, B. Courtney Barrer, S. Curaba, R. Davies, D. Defrère, A. Delboulbé, F. Delplancke , et al. (115 additional authors not shown)

Abstract: GRAVITY+ improves by orders of magnitude the sensitivity, sky-coverage and contrast of the Very Large Telescope Interferometer (VLTI). A central part of this project is the development of Gravity Plus Adaptive Optics (GPAO), a dedicated high-order and laser-guide star Adaptive Optics (AO) system for VLTI. GPAO consists of four state-of-the-art AO systems equipping all 8m-class Unit Telescopes (UTs… ▽ More GRAVITY+ improves by orders of magnitude the sensitivity, sky-coverage and contrast of the Very Large Telescope Interferometer (VLTI). A central part of this project is the development of Gravity Plus Adaptive Optics (GPAO), a dedicated high-order and laser-guide star Adaptive Optics (AO) system for VLTI. GPAO consists of four state-of-the-art AO systems equipping all 8m-class Unit Telescopes (UTs) for the wavefront correction of the VLTI instruments. It offers both visible and infrared Natural Guide Star (NGS) and Laser Guide Star (LGS) operations. The paper presents the design, operations and performances of GPAO. We illustrate the improvement brought by GPAO with interferometric observations obtained during the commissioning of the NGS mode end-2024. These science results include the first optical interferometry observations of a redshift $z\sim4$ quasar, the spectroscopy of a cool brown-dwarf with magnitude $K\sim 21.0$, the first observations of a Class I young star with GRAVITY, and the first sub-micro arcsecond differential astrometry in the optical. Together with the entire GRAVITY+ project, the implementation of GPAO is a true paradigm shift for observing the optical Universe at very high angular resolution. △ Less

Submitted 11 December, 2025; v1 submitted 25 September, 2025; originally announced September 2025.

Comments: Accepted in A&A

Journal ref: A&A 707, A115 (2026)

Constraints on the Orbit of the Young Substellar Companion GQ Lup B from High-Resolution Spectroscopy and VLTI/GRAVITY Astrometry

Authors: Vidya Venkatesan, S. Blunt, J. J. Wang, S. Lacour, G. -D. Marleau, G. A. L. Coleman, L. Guerrero, W. O. Balmer, L. Pueyo, T. Stolker, J. Kammerer, N. Pourré, M. Nowak, E. Rickman, A. Sivaramakrishnan, D. Sing, K. Wagner, A. -M. Lagrange, R. Abuter, A. Amorim, R. Asensio-Torres, J. -P. Berger, H. Beust, A. Boccaletti, M. Bonnefoy , et al. (77 additional authors not shown)

Abstract: Understanding the orbits of giant planets is critical for testing planet formation models, particularly at wide separations greater than 10 au where traditional core accretion becomes inefficient. However, constraining orbits at these separations has been challenging because of sparse orbital coverage and degeneracies in the orbital parameters. We use existing high-resolution spectroscopic measure… ▽ More Understanding the orbits of giant planets is critical for testing planet formation models, particularly at wide separations greater than 10 au where traditional core accretion becomes inefficient. However, constraining orbits at these separations has been challenging because of sparse orbital coverage and degeneracies in the orbital parameters. We use existing high-resolution spectroscopic measurements from CRIRES+ (R ~ 100000), astrometric data from SPHERE, NACO, and ALMA, and new high-precision GRAVITY astrometry to refine the orbit of GQ Lup B, a ~30 M_J companion at ~100 au, in a system that also hosts a circumstellar disk and a wide companion, GQ Lup C. Including radial velocity data significantly improves orbital constraints by breaking the degeneracy between inclination and eccentricity that affects astrometry-only fits for long-period companions. This work is among the first to combine high-precision astrometry with the companion's relative radial velocity to achieve improved orbital constraints. The eccentricity is refined from e = 0.47 (+0.14, -0.16) with GRAVITY alone to e = 0.35 (+0.10, -0.09) when RVs and GRAVITY data are combined. The orbit is misaligned by 63 (+6, -14) deg relative to the circumstellar disk and 52 (+19, -24) deg relative to the host star spin axis, and is more consistent (34 (+6, -13) deg) with the inclination of the wide tertiary companion GQ Lup C disk. These results support a formation scenario for GQ Lup B consistent with cloud fragmentation and highlight the power of combining companion RV constraints with interferometric astrometry to probe the dynamics and formation of wide-orbit substellar companions. △ Less

Submitted 24 September, 2025; originally announced September 2025.

Comments: 15 pages, 5 figures, Accepted to ApJ

Spatially resolved broad line region in a quasar at z=4: Dynamical black hole mass and prominent outflow

Authors: GRAVITY+ Collaboration, K. Abd El Dayem, N. Aimar, A. Berdeu, J. -P. Berger, G. Bourdarot, P. Bourget, W. Brandner, Y. Cao, C. Correia, S. Cuevas Cardona, R. Davies, D. Defrère, A. Drescher, A. Eckart, F. Eisenhauer, M. Fabricius, A. Farah, H. Feuchtgruber, N. M. Förster Schreiber, A. Foschi, P. Garcia, R. Garcia Lopez, R. Genzel, S. Gillessen , et al. (70 additional authors not shown)

Abstract: We present the first near-infrared interferometric data of a QSO at z=4. The K-band observations were performed with GRAVITY+ on the VLTI using all four UTs, detecting a differential phase signal that traces the spatially resolved kinematics for both the \hb\ and \hg\ lines in the broad line region. We fit the two lines simultaneously with an updated model that includes distinct rotating and conic… ▽ More We present the first near-infrared interferometric data of a QSO at z=4. The K-band observations were performed with GRAVITY+ on the VLTI using all four UTs, detecting a differential phase signal that traces the spatially resolved kinematics for both the \hb\ and \hg\ lines in the broad line region. We fit the two lines simultaneously with an updated model that includes distinct rotating and conical outflowing components. For the best fit model, more than 80\% of the \hi\ line emission from the BLR originates in an outflow with a velocity up to $10^4$ km s$^{-1}$. This is oriented so that our line of sight is along an edge of the conical structure, which produces the prominent blue wing on the line profile. A combination of anisotropic line emission and mid-plane opacity lead to the single-sided phase signal. The model is able to qualitatively match both the outflowing \civ\ line profile and the systemic \oi\ fluorescent emission. The derived black hole mass of $8\times10^8$ M$_\odot$ is the highest redshift black hole mass measurement to date obtained directly from BLR dynamics. It is an order of magnitude lower than that inferred from various single epoch scaling relations, and implies that the accretion is highly super-Eddington. With reference to recent simulations, the data suggest that this QSO is emitting close to its radiative limit in a regime where strong outflows are expected around a polar conical region. △ Less

Submitted 12 December, 2025; v1 submitted 17 September, 2025; originally announced September 2025.

Comments: accepted by A&A (15 pages, 9 figures)

Journal ref: A&A 706, A99 (2026)

The mid-infrared spectrum of $β$ Pictoris b. First VLTI/MATISSE interferometric observations of an exoplanet

Authors: M. Houllé, F. Millour, P. Berio, J. Scigliuto, S. Lacour, B. Lopez, F. Allouche, J. -C. Augereau, D. Blain, M. Bonnefoy, M. Carbillet, G. Chauvin, J. Leftley, A. Matter, J. Milli, P. Mollière, E. Nasedkin, M. Nowak, P. Palma-Bifani, É. Pantin, P. Priolet, M. Ravet, J. Woillez, W. Balmer, P. Boley , et al. (24 additional authors not shown)

Abstract: Few spectra of directly-imaged exoplanets have been obtained in the mid-infrared (> 3 $μ$m). This region is particularly rich in molecular spectral signatures, whose measurements can help recover atmospheric parameters and provide a better understanding of giant planet formation and atmospheric dynamics. In the past years, exoplanet interferometry with the VLTI/GRAVITY instrument has provided medi… ▽ More Few spectra of directly-imaged exoplanets have been obtained in the mid-infrared (> 3 $μ$m). This region is particularly rich in molecular spectral signatures, whose measurements can help recover atmospheric parameters and provide a better understanding of giant planet formation and atmospheric dynamics. In the past years, exoplanet interferometry with the VLTI/GRAVITY instrument has provided medium-resolution spectra of a dozen substellar companions in the near infrared. The 100-meter interferometric baselines allow for the stellar and planetary signals to be efficiently disentangled at close angular separations (< 0.3''). We aim to extend this technique to the mid-infrared using MATISSE, the VLTI's mid-infrared spectro-interferometer. We take advantage of the fringe tracking and off-axis pointing capabilities recently brought by the GRA4MAT upgrade. Using this new mode, we observed the giant planet $β$ Pictoris b in L and M bands (2.75-5 $μ$m) at a spectral resolution of 500. We developed a method to correct chromatic dispersion and non-common paths effects in the fringe phase and modelled the planet astrometry and stellar contamination. We obtained a high-signal-to-noise spectrum of $β$ Pictoris b, showing the planet continuum in L (for the first time) and M bands, which contains broad absorption features of H$_2$O and CO. In conjunction with a new GRAVITY spectrum, we modelled it with the ForMoSA nested sampling tool and the Exo-REM grid of atmospheric models, and found a solar carbon-to-oxygen ratio in the planet atmosphere. This study opens the way to the characterization of fainter and closer-in planets with MATISSE, which could complement the JWST at angular separations too close for it to obtain exoplanet spectra. Starting in 2025, the new adaptive optics system brought by the GRAVITY+ upgrade will further extend the detection limits of MATISSE. △ Less

Submitted 9 December, 2025; v1 submitted 25 August, 2025; originally announced August 2025.

Comments: 21 pages, 17 figures. Final version published in A&A

Journal ref: A&A 704, A182 (2025)

Direct imaging discovery of a young giant planet orbiting on Solar System scales

Authors: T. Stolker, M. Samland, L. B. F. M. Waters, M. E. van den Ancker, W. O. Balmer, S. Lacour, M. L. Sitko, J. J. Wang, M. Nowak, A. -L. Maire, J. Kammerer, G. P. P. L. Otten, R. Abuter, A. Amorim, M. Benisty, J. -P. Berger, H. Beust, S. Blunt, A. Boccaletti, M. Bonnefoy, H. Bonnet, M. S. Bordoni, G. Bourdarot, W. Brandner, F. Cantalloube , et al. (80 additional authors not shown)

Abstract: HD 135344 AB is a young visual binary system that is best known for the protoplanetary disk around the secondary star. The circumstellar environment of the A0-type primary star, on the other hand, is already depleted. HD 135344 A is therefore an ideal target for the exploration of recently formed giant planets because it is not obscured by dust. We searched for and characterized substellar compani… ▽ More HD 135344 AB is a young visual binary system that is best known for the protoplanetary disk around the secondary star. The circumstellar environment of the A0-type primary star, on the other hand, is already depleted. HD 135344 A is therefore an ideal target for the exploration of recently formed giant planets because it is not obscured by dust. We searched for and characterized substellar companions to HD 135344 A down to separations of about 10 au. We observed HD 135344 A with VLT/SPHERE in the $H23$ and $K12$ bands and obtained $YJ$ and $YJH$ spectroscopy. In addition, we carried out VLTI/GRAVITY observations for the further astrometric and spectroscopic confirmation of a detected companion. We discovered a close-in young giant planet, HD 135344 Ab, with a mass of about 10 $M_\mathrm{J}$. The multi-epoch astrometry confirms the bound nature based on common parallax and common proper motion. This firmly rules out the scenario of a non-stationary background star. The semi-major axis of the planetary orbit is approximately 15-20 au, and the photometry is consistent with that of a mid L-type object. The inferred atmospheric and bulk parameters further confirm the young and planetary nature of the companion. HD 135344 Ab is one of the youngest directly imaged planets that has fully formed and orbits on Solar System scales. It is a valuable target for studying the early evolution and atmosphere of a giant planet that could have formed in the vicinity of the snowline. △ Less

Submitted 8 July, 2025; originally announced July 2025.

Comments: 13 pages, 11 figures, accepted for publication in A&A

Orbit and atmosphere of HIP 99770 b through the eyes of VLTI/GRAVITY

Authors: T. O. Winterhalder, J. Kammerer, S. Lacour, A. Mérand, M. Nowak, T. Stolker, W. O. Balmer, G. -D. Marleau, R. Abuter, A. Amorim, R. Asensio-Torres, J. -P. Berger, H. Beust, S. Blunt, M. Bonnefoy, H. Bonnet, M. S. Bordoni, G. Bourdarot, W. Brandner, F. Cantalloube, P. Caselli, B. Charnay, G. Chauvin, A. Chavez, E. Choquet , et al. (70 additional authors not shown)

Abstract: Context: Inferring the likely formation channel of giant exoplanets and brown dwarf companions from orbital and atmospheric observables remains a formidable challenge. Further and more precise directly measured dynamical masses of these companions are required to inform and gauge formation, evolutionary, and atmospheric models. We present an updated study of HIP 99770 b based on observations condu… ▽ More Context: Inferring the likely formation channel of giant exoplanets and brown dwarf companions from orbital and atmospheric observables remains a formidable challenge. Further and more precise directly measured dynamical masses of these companions are required to inform and gauge formation, evolutionary, and atmospheric models. We present an updated study of HIP 99770 b based on observations conducted with VLTI/GRAVITY. Aims: Combining the new data with previous observations from the literature, we characterise HIP 99770 b to better constrain its orbit, dynamical mass, and atmospheric properties, as well as to shed light on its likely formation channel. Methods: We ran a renewed orbit fit to further constrain the dynamical mass of the companion and the orbit solution. We also analysed the GRAVITY K-band spectrum, placing it into context with literature data, and extracting magnitude, age, spectral type, bulk properties and atmospheric characteristics of HIP 99770 b. Results: We detected the companion at a radial separation of $417\,\mathrm{mas}$ from its host. The new orbit fit yields a dynamical mass of $17_{-5}^{+6}\,\mathrm{M}_\mathrm{Jup}$ and an eccentricity of $0.31_{-0.12}^{+0.06}$. We also find that additional relative astrometry epochs in the future will not enable further constraints on the dynamical mass due to the dominating relative uncertainty on the Hipparcos-Gaia proper motion anomaly. The publication of Gaia DR4 will likely ease this predicament. We find that the companion is consistent with spectral type L8 and exhibits a potential metal enrichment in its atmosphere. Conclusions: These results do not yet allow for a definite inference of the companion's formation channel. Nevertheless, the new constraints on its bulk properties and the additional GRAVITY spectrum presented here will aid future efforts to determine the formation history of HIP 99770 b. △ Less

Submitted 30 June, 2025; originally announced July 2025.

Comments: Accepted for publication in Astronomy and Astrophysics

Exploring the presence of a fifth force at the Galactic Center

Authors: The GRAVITY Collaboration, K. Abd El Dayem, R. Abuter, N. Aimar, P. Amaro Seoane, A. Amorim, J. P. Berger, H. Bonnet, G. Bourdarot, W. Brandner, V. Cardoso, Y. Clénet, R. Davies, P. T. de Zeeuw, A. Drescher, A. Eckart, F. Eisenhauer, H. Feuchtgruber, G. Finger, N. M. Förster Schreiber, A. Foschi, P. Garcia, E. Gendron, R. Genzel, S. Gillessen , et al. (38 additional authors not shown)

Abstract: Aims: The presence of a Yukawa-like correction to Newtonian gravity is investigated at the Galactic Center, leading to a new upper limit for the intensity of such a correction. Methods: We perform a Markov Chain Monte Carlo analysis using the astrometric and spectroscopic data of star S$2$ collected at the Very Large Telescope by GRAVITY, NACO and SINFONI instruments, covering the period from… ▽ More Aims: The presence of a Yukawa-like correction to Newtonian gravity is investigated at the Galactic Center, leading to a new upper limit for the intensity of such a correction. Methods: We perform a Markov Chain Monte Carlo analysis using the astrometric and spectroscopic data of star S$2$ collected at the Very Large Telescope by GRAVITY, NACO and SINFONI instruments, covering the period from $1992$ to $2022$. Results: The precision of the GRAVITY instrument allows us to derive the most stringent upper limit at the Galactic Center for the intensity of the Yukawa contribution ($\propto \, αe^{- λr}$) to be $|α| < 0.003$ for a scale length $λ= 3 \cdot 10^{13}\, \rm m\, (\sim 200 \, \rm AU)$. This improves by roughly one order of magnitude all estimates obtained in previous works. △ Less

Submitted 24 February, 2026; v1 submitted 3 April, 2025; originally announced April 2025.

Comments: Updated to match published version. Accepted on A&A

Journal ref: A&A 698, L15 (2025)

The cool brown dwarf Gliese 229 B is a close binary

Authors: Jerry W. Xuan, A. Mérand, W. Thompson, Y. Zhang, S. Lacour, D. Blakely, D. Mawet, R. Oppenheimer, J. Kammerer, K. Batygin, A. Sanghi, J. Wang, J. -B. Ruffio, M. C. Liu, H. Knutson, W. Brandner, A. Burgasser, E. Rickman, R. Bowens-Rubin, M. Salama, W. Balmer, S. Blunt, G. Bourdarot, P. Caselli, G. Chauvin , et al. (54 additional authors not shown)

Abstract: Owing to their similarities with giant exoplanets, brown dwarf companions of stars provide insights into the fundamental processes of planet formation and evolution. From their orbits, several brown dwarf companions are found to be more massive than theoretical predictions given their luminosities and the ages of their host stars (e.g. Brandt et al. 2021, Cheetham et al. 2018, Li et al. 2023). Eit… ▽ More Owing to their similarities with giant exoplanets, brown dwarf companions of stars provide insights into the fundamental processes of planet formation and evolution. From their orbits, several brown dwarf companions are found to be more massive than theoretical predictions given their luminosities and the ages of their host stars (e.g. Brandt et al. 2021, Cheetham et al. 2018, Li et al. 2023). Either the theory is incomplete or these objects are not single entities. For example, they could be two brown dwarfs each with a lower mass and intrinsic luminosity (Brandt et al. 2021, Howe et al. 2024). The most problematic example is Gliese 229 B (Nakajima et al. 1995, Oppenheimer et al. 1995), which is at least 2-6 times less luminous than model predictions given its dynamical mass of $71.4\pm0.6$ Jupiter masses ($M_{\rm Jup}$) (Brandt et al. 2021). We observed Gliese 229 B with the GRAVITY interferometer and, separately, the CRIRES+ spectrograph at the Very Large Telescope. Both sets of observations independently resolve Gliese 229 B into two components, Gliese 229 Ba and Bb, settling the conflict between theory and observations. The two objects have a flux ratio of $0.47\pm0.03$ at a wavelength of 2 $μ$m and masses of $38.1\pm1.0$ and $34.4\pm1.5$ $M_{\rm Jup}$, respectively. They orbit each other every 12.1 days with a semimajor axis of 0.042 astronomical units (AU). The discovery of Gliese 229 BaBb, each only a few times more massive than the most massive planets, and separated by 16 times the Earth-moon distance, raises new questions about the formation and prevalence of tight binary brown dwarfs around stars. △ Less

Submitted 15 October, 2024; originally announced October 2024.

Comments: Published in Nature. The Version of Record of this article is located at https://www.nature.com/articles/s41586-024-08064-x

Investigating 39 Galactic Wolf-Rayet stars with VLTI/GRAVITY: Uncovering A Long Period Binary Desert

Authors: K. Deshmukh, H. Sana, A. Mérand, E. Bordier, N. Langer, J. Bodensteiner, K. Dsilva, A. J. Frost, E. Gosset, J. -B. Le Bouquin, R. R. Lefever, L. Mahy, L. R. Patrick, M. Reggiani, A. A. C. Sander, T. Shenar, F. Tramper, J. I. Villaseñor, I. Waisberg

Abstract: Wolf-Rayet stars (WRs) are one of the final evolutionary stages of massive stars and immediate progenitors of stellar-mass black holes. Their multiplicity forms an important anchor point in single and binary population models for predicting gravitational-wave progenitors. Recent spectroscopic campaigns have suggested incompatible multiplicity fractions and period distributions for N- and C-rich Ga… ▽ More Wolf-Rayet stars (WRs) are one of the final evolutionary stages of massive stars and immediate progenitors of stellar-mass black holes. Their multiplicity forms an important anchor point in single and binary population models for predicting gravitational-wave progenitors. Recent spectroscopic campaigns have suggested incompatible multiplicity fractions and period distributions for N- and C-rich Galactic WRs (WNs and WCs) at short as well as long orbital periods, in contradiction with evolutionary model predictions. In this work, we employed infrared interferometry using the $K$-band instrument GRAVITY at the VLTI to investigate the multiplicity of WRs at long periods and explore the nature of their companions. We present a survey of 39 Galactic WRs, including 11 WN, 15 WC and 13 H-rich WN (WNh) stars. We detected wide companions with GRAVITY for only four stars: WR 48, WR 89, WR 93 and WR 115. Combining with spectroscopic studies, we arrived at multiplicity fractions of $f^{\rm WN}_{\rm obs} = 0.55\pm0.15$, $f^{\rm WC}_{\rm obs} = 0.40\pm0.13$ and $f^{\rm WNh}_{\rm obs} = 0.23\pm0.12$. In addition, we also found other features in the GRAVITY dataset such as (i) a diffuse extended component in over half the WR sample; (ii) five known spectroscopic binaries resolved in differential phase data and (iii) spatially resolved winds in four stars: WR 16, WR 31a, WR 78 and WR 110. Our survey reveals a lack of intermediate (few 100s d) and long- (few years to decades) period WR systems. The 200-d peak in the period distributions of WR+OB and BH+OB binaries predicted by Case B mass-transfer binary evolution models is not seen in our data. The rich companionship of their O-type progenitors in this separation range suggest that the WR progenitor stars expand and interact with their companions, most likely through unstable mass-transfer, resulting in either a short-period system or a merger. △ Less

Submitted 25 October, 2024; v1 submitted 23 September, 2024; originally announced September 2024.

Comments: Accepted in A&A

Journal ref: A&A 692, A109 (2024)

Observations of Microlensed Images with Dual-field Interferometry: On-sky Demonstration and Prospects

Authors: P. Mroz, S. Dong, A. Merand, J. Shangguan, J. Woillez, A. Gould, A. Udalski, F. Eisenhauer, Y. -H. Ryu, Z. Wu, Z. Liu, H. Yang, G. Bourdarot, D. Defrere, A. Drescher, M. Fabricius, P. Garcia, R. Genzel, S. Gillessen, S. F. Honig, L. Kreidberg, J. -B. Le Bouquin, D. Lutz, F. Millour, T. Ott , et al. (35 additional authors not shown)

Abstract: Interferometric observations of gravitational microlensing events offer an opportunity for precise, efficient, and direct mass and distance measurements of lensing objects, especially those of isolated neutron stars and black holes. However, such observations have previously been possible for only a handful of extremely bright events. The recent development of a dual-field interferometer, GRAVITY… ▽ More Interferometric observations of gravitational microlensing events offer an opportunity for precise, efficient, and direct mass and distance measurements of lensing objects, especially those of isolated neutron stars and black holes. However, such observations have previously been possible for only a handful of extremely bright events. The recent development of a dual-field interferometer, GRAVITY Wide, has made it possible to reach out to significantly fainter objects and increase the pool of microlensing events amenable to interferometric observations by two orders of magnitude. Here, we present the first successful observation of a microlensing event with GRAVITY Wide and the resolution of microlensed images in the event OGLE-2023-BLG-0061/KMT-2023-BLG-0496. We measure the angular Einstein radius of the lens with subpercent precision, $θ_{\rm E} = 1.280 \pm 0.009$ mas. Combined with the microlensing parallax detected from the event light curve, the mass and distance to the lens are found to be $0.472 \pm 0.012\,M_{\odot}$ and $1.81 \pm 0.05$ kpc, respectively. We present the procedure for the selection of targets for interferometric observations and discuss possible systematic effects affecting GRAVITY Wide data. This detection demonstrates the capabilities of the new instrument, and it opens up completely new possibilities for the follow-up of microlensing events and future routine discoveries of isolated neutron stars and black holes. △ Less

Submitted 3 January, 2025; v1 submitted 18 September, 2024; originally announced September 2024.

Comments: accepted for publication in ApJ

Journal ref: ApJ 980, 47 (2025)

The GRAVITY young stellar object survey XIV : Investigating the magnetospheric accretion-ejection processes in S CrA N

Authors: GRAVITY Collaboration, H. Nowacki, K. Perraut, L. Labadie, J. Bouvier, C. Dougados, M. Benisty, J. A. Wojtczak, A. Soulain, E. Alecian, W. Brandner, A. Caratti o Garatti, R. Garcia Lopez, V. Ganci, J. Sánchez-Bermúdez, J. -P. Berger, G. Bourdarot, P. Caselli, Y. Clénet, R. Davies, A. Drescher, A. Eckart, F. Eisenhauer, M. Fabricius, H. Feuchtgruber , et al. (31 additional authors not shown)

Abstract: The dust- and gas-rich protoplanetary disks around young stellar systems play a key role in star and planet formation. While considerable progress has recently been made in probing these disks on large scales of a few tens of astronomical units (au), the central au needs to be more investigated. We aim at unveiling the physical processes at play in the innermost regions of the strongly accreting T… ▽ More The dust- and gas-rich protoplanetary disks around young stellar systems play a key role in star and planet formation. While considerable progress has recently been made in probing these disks on large scales of a few tens of astronomical units (au), the central au needs to be more investigated. We aim at unveiling the physical processes at play in the innermost regions of the strongly accreting T Tauri Star S CrA N by means of near-infrared interferometric observations. The K-band continuum emission is well reproduced with an azimuthally-modulated dusty ring. As the star alone cannot explain the size of this sublimation front, we propose that magnetospheric accretion is an important dust-heating mechanism leading to this continuum emission. The differential analysis of the Hydrogen Br$γ$ line is in agreement with radiative transfer models combining magnetospheric accretion and disk winds. Our observations support an origin of the Br$γ$ line from a combination of (variable) accretion-ejection processes in the inner disk region. △ Less

Submitted 5 August, 2024; originally announced August 2024.

Journal ref: A&A 690, A123 (2024)

Estimation of the lateral mis-registrations of the GRAVITY + adaptive optics system

Authors: Anthony Berdeu, H. Bonnet, J. -B. Le Bouquin, C. Édouard, T. Gomes, P. Shchekaturov, R. Dembet, T. Paumard, S. Oberti, J. Kolb, F. Millour, P. Berio, O. Lai, F. Eisenhauer, P. Garcia, C. Straubmeier, L. Kreidberg, S. Hönig, D. Defrère

Abstract: Context. The GRAVITY+ upgrade implies a complete renewal of its adaptive optics (AO) systems. Its complex design, featuring moving components between the deformable mirrors and the wavefront sensors, requires the monitoring and auto-calibrating of the lateral mis-registrations of the system while in operation. Aims. For preset and target acquisition, large lateral registration errors must be asses… ▽ More Context. The GRAVITY+ upgrade implies a complete renewal of its adaptive optics (AO) systems. Its complex design, featuring moving components between the deformable mirrors and the wavefront sensors, requires the monitoring and auto-calibrating of the lateral mis-registrations of the system while in operation. Aims. For preset and target acquisition, large lateral registration errors must be assessed in open loop to bring the system to a state where the AO loop closes. In closed loop, these errors must be monitored and corrected, without impacting the science. Methods. With respect to the first requirement, our method is perturbative, with two-dimensional modes intentionally applied to the system and correlated to a reference interaction matrix. For the second requirement, we applied a non-perturbative approach that searches for specific patterns in temporal correlations in the closed loop telemetry. This signal is produced by the noise propagation through the AO loop. Results. Our methods were validated through simulations and on the GRAVITY+ development bench. The first method robustly estimates the lateral mis-registrations, in a single fit and with a sub-subaperture resolution while in an open loop. The second method is not absolute, but it does successfully bring the system towards a negligible mis-registration error, with a limited turbulence bias. Both methods proved to robustly work on a system still under development and not fully characterised. Conclusions. Tested with Shack-Hartmann wavefront sensors, the proposed methods are versatile and easily adaptable to other AO instruments, such as the pyramid, which stands as a baseline for all future AO systems. The non-perturbative method, not relying on an interaction matrix model and being sparse in the Fourier domain, is particularly suitable to the next generation of AO systems for extremely large telescopes that will present an unprecedented level of complexity and numbers of actuators. △ Less

Submitted 10 July, 2024; originally announced July 2024.

Journal ref: Astronomy and Astrophysics - A&A, 2024, 687, pp.A157

High contrast at short separation with VLTI/GRAVITY: Bringing Gaia companions to light

Authors: N. Pourré, T. O. Winterhalder, J. -B. Le Bouquin, S. Lacour, A. Bidot, M. Nowak, A. -L. Maire, D. Mouillet, C. Babusiaux, J. Woillez, R. Abuter, A. Amorim, R. Asensio-Torres, W. O. Balmer, M. Benisty, J. -P. Berger, H. Beust, S. Blunt, A. Boccaletti, M. Bonnefoy, H. Bonnet, M. S. Bordoni, G. Bourdarot, W. Brandner, F. Cantalloube , et al. (151 additional authors not shown)

Abstract: Since 2019, GRAVITY has provided direct observations of giant planets and brown dwarfs at separations of down to 95 mas from the host star. Some of these observations have provided the first direct confirmation of companions previously detected by indirect techniques (astrometry and radial velocities). We want to improve the observing strategy and data reduction in order to lower the inner working… ▽ More Since 2019, GRAVITY has provided direct observations of giant planets and brown dwarfs at separations of down to 95 mas from the host star. Some of these observations have provided the first direct confirmation of companions previously detected by indirect techniques (astrometry and radial velocities). We want to improve the observing strategy and data reduction in order to lower the inner working angle of GRAVITY in dual-field on-axis mode. We also want to determine the current limitations of the instrument when observing faint companions with separations in the 30-150 mas range. To improve the inner working angle, we propose a fiber off-pointing strategy during the observations to maximize the ratio of companion-light-to-star-light coupling in the science fiber. We also tested a lower-order model for speckles to decouple the companion light from the star light. We then evaluated the detection limits of GRAVITY using planet injection and retrieval in representative archival data. We compare our results to theoretical expectations. We validate our observing and data-reduction strategy with on-sky observations; first in the context of brown dwarf follow-up on the auxiliary telescopes with HD 984 B, and second with the first confirmation of a substellar candidate around the star Gaia DR3 2728129004119806464. With synthetic companion injection, we demonstrate that the instrument can detect companions down to a contrast of $8\times 10^{-4}$ ($Δ\mathrm{K}= 7.7$ mag) at a separation of 35 mas, and a contrast of $3\times 10^{-5}$ ($Δ\mathrm{K}= 11$ mag) at 100 mas from a bright primary (K<6.5), for 30 min exposure time. With its inner working angle and astrometric precision, GRAVITY has a unique reach in direct observation parameter space. This study demonstrates the promising synergies between GRAVITY and Gaia for the confirmation and characterization of substellar companions. △ Less

Submitted 6 June, 2024; originally announced June 2024.

Comments: 16 pages, 14 figures. Submitted to A&A

A new dimension in the variability of AGB stars: convection patterns size changes with pulsation

Authors: A. Rosales-Guzmán, J. Sanchez-Bermudez, C. Paladini, B. Freytag, M. Wittkowski, A. Alberdi, F. Baron, J. -P. Berger, A. Chiavassa, S. Höfner, A. Jorissen, P. Kervella, J. -B. Le Bouquin, P. Marigo, M. Montargès, M. Trabucchi, S. Tsvetkova, R. Schödel, S. Van Eck

Abstract: Stellar convection plays an important role in atmospheric dynamics, wind formation and the mass-loss processes in Asymptotic Giant Branch (AGB) stars. However, a direct characterization of convective surface structures in terms of size, contrast, and life-span is quite challenging. Spatially resolving these features requires the highest angular resolution. In this work, we aim at characterizing th… ▽ More Stellar convection plays an important role in atmospheric dynamics, wind formation and the mass-loss processes in Asymptotic Giant Branch (AGB) stars. However, a direct characterization of convective surface structures in terms of size, contrast, and life-span is quite challenging. Spatially resolving these features requires the highest angular resolution. In this work, we aim at characterizing the size of convective structures on the surface of the O-rich AGB star R Car to test different theoretical predictions, based on mixing-length theory from solar models. We used infrared low-spectral resolution (R~35) interferometric data in the H-band (~1.76 $μ$m) with the instrument PIONIER at the Very Large Telescope Interferometer (VLTI) to image the star's surface at two epochs separated by ~6 years. Using a power spectrum analysis, we estimate the horizontal size of the structures on the surface of R Car. The sizes of the stellar disk, at different phases of a pulsation cycle, were obtained using parametric model-fitting in the Fourier domain. Our analysis supports that the sizes of the structures in R Car are correlated with variations of the pressure scale height in the atmosphere of the target, as predicted by theoretical models based on solar convective processes. We observe that these structures grow in size when the star expands within a pulsation cycle. While the information is still scarce, this observational finding highlights the role of convection in the dynamics of those objects. New interferometric imaging campaigns with the renewed capabilities of the VLTI are envisioned to expand our analysis to a larger sample of objects. △ Less

Submitted 16 May, 2024; originally announced May 2024.

Comments: Accepted by A&A on 15/05/2024, 11 pages, 7 figures

Journal ref: A&A 688, A124 (2024)

A magnetic massive star has experienced a stellar merger

Authors: A. J. Frost, H. Sana, L. Mahy, G. Wade, J. Barron, J. -B. Le Bouquin, A. Mérand, F. R. N. Schneider, T. Shenar, R. H. Barbá, D. M. Bowman, M. Fabry, A. Farhang, P. Marchant, N. I. Morrell, J. V. Smoker

Abstract: Massive stars (those larger than 8 solar masses at formation) have radiative envelopes that cannot sustain a dynamo, the mechanism that produces magnetic fields in lower-mass stars. Despite this, approximately 7\% of massive stars have observed magnetic fields, the origin of which is debated. We used multi-epoch interferometric and spectroscopic observations to characterize HD 148937, a binary sys… ▽ More Massive stars (those larger than 8 solar masses at formation) have radiative envelopes that cannot sustain a dynamo, the mechanism that produces magnetic fields in lower-mass stars. Despite this, approximately 7\% of massive stars have observed magnetic fields, the origin of which is debated. We used multi-epoch interferometric and spectroscopic observations to characterize HD 148937, a binary system of two massive stars. We found that only one star is magnetic and that it appears younger than its companion. The system properties and a surrounding bipolar nebula can be reproduced with a model in which two stars merged (in a previous triple system) to produce the magnetic massive star. Our results provide observational evidence that magnetic fields form in at least some massive stars through stellar mergers. △ Less

Submitted 15 April, 2024; originally announced April 2024.

Comments: Full paper with supplementary materials. 59 pages, 18 figures

Journal ref: Science, 11 Apr 2024, Vol 384, Issue 6692, pp. 214-217

Four-of-a-kind? Comprehensive atmospheric characterisation of the HR 8799 planets with VLTI/GRAVITY

Authors: E. Nasedkin, P. Mollière, S. Lacour, M. Nowak, L. Kreidberg, T. Stolker, J. J. Wang, W. O. Balmer, J. Kammerer, J. Shangguan, R. Abuter, A. Amorim, R. Asensio-Torres, M. Benisty, J. -P. Berger, H. Beust, S. Blunt, A. Boccaletti, M. Bonnefoy, H. Bonnet, M. S. Bordoni, G. Bourdarot, W. Brandner, F. Cantalloube, P. Caselli , et al. (73 additional authors not shown)

Abstract: With four companions at separations from 16 to 71 au, HR 8799 is a unique target for direct imaging, presenting an opportunity for the comparative study of exoplanets with a shared formation history. Combining new VLTI/GRAVITY observations obtained within the ExoGRAVITY program with archival data, we perform a systematic atmospheric characterisation of all four planets. We explore different levels… ▽ More With four companions at separations from 16 to 71 au, HR 8799 is a unique target for direct imaging, presenting an opportunity for the comparative study of exoplanets with a shared formation history. Combining new VLTI/GRAVITY observations obtained within the ExoGRAVITY program with archival data, we perform a systematic atmospheric characterisation of all four planets. We explore different levels of model flexibility to understand the temperature structure, chemistry and clouds of each planet using both petitRADTRANS atmospheric retrievals and fits to self-consistent radiative-convective equilibrium models. Using Bayesian Model Averaging to combine multiple retrievals, we find that the HR 8799 planets are highly enriched in metals, with [M/H] $\gtrsim$1, and have stellar to super-stellar C/O ratios. The C/O ratio increases with increasing separation from $0.55^{+0.12}_{-0.10}$ for d to $0.78^{+0.03}_{-0.04}$ for b, with the exception of the innermost planet which has a C/O ratio of $0.87\pm0.03$. By retrieving a quench pressure and using a disequilibrium chemistry model we derive vertical mixing strengths compatible with predictions for high-metallicity, self-luminous atmospheres. Bayesian evidence comparisons strongly favour the presence of HCN in HR 8799 c and e, as well as CH$_{4}$ in HR 8799 c, with detections at $>5σ$ confidence. All of the planets are cloudy, with no evidence for patchiness. The clouds of c, d and e are best fit by silicate clouds lying above a deep iron cloud layer, while the clouds of the cooler HR 8799 b are more likely composed of Na$_{2}$S. With well defined atmospheric properties, future exploration of this system is well positioned to unveil further detail in these planets, extending our understanding of the composition, structure, and formation history of these siblings. △ Less

Submitted 17 July, 2024; v1 submitted 4 April, 2024; originally announced April 2024.

Comments: 45 pages, 25 figures, accepted by Astronomy & Astrophysics

Journal ref: A&A 687, A298 (2024)

Combining Gaia and GRAVITY: Characterising five new Directly Detected Substellar Companions

Authors: T. O. Winterhalder, S. Lacour, A. Mérand, A. -L. Maire, J. Kammerer, T. Stolker, N. Pourré, C. Babusiaux, R. Abuter, A. Amorim, R. Asensio-Torres, W. O. Balmer, M. Benisty, J. -P. Berger, H. Beust, S. Blunt, A. Boccaletti, M. Bonnefoy, H. Bonnet, M. S. Bordoni, G. Bourdarot, W. Brandner, F. Cantalloube, P. Caselli, B. Charnay , et al. (74 additional authors not shown)

Abstract: Precise mass constraints are vital for the characterisation of brown dwarfs and exoplanets. Here we present how the combination of data obtained by Gaia and GRAVITY can help enlarge the sample of substellar companions with measured dynamical masses. We show how the Non-Single-Star (NSS) two-body orbit catalogue contained in Gaia DR3 can be used to inform high-angular-resolution follow-up observati… ▽ More Precise mass constraints are vital for the characterisation of brown dwarfs and exoplanets. Here we present how the combination of data obtained by Gaia and GRAVITY can help enlarge the sample of substellar companions with measured dynamical masses. We show how the Non-Single-Star (NSS) two-body orbit catalogue contained in Gaia DR3 can be used to inform high-angular-resolution follow-up observations with GRAVITY. Applying the method presented in this work to eight Gaia candidate systems, we detect all eight predicted companions, seven of which were previously unknown and five are of a substellar nature. Among the sample is Gaia DR3 2728129004119806464 B, which - detected at an angular separation of (34.01 $\pm$ 0.15) mas from the host - is the closest substellar companion ever imaged. This translates to a semi-major axis of (0.938 $\pm$ 0.023) AU. WT 766 B, detected at a greater angular separation, was confirmed to be on an orbit exhibiting an even smaller semi-major axis of (0.676 $\pm$ 0.008) AU. The GRAVITY data were then used to break the host-companion mass degeneracy inherent to the Gaia NSS orbit solutions as well as to constrain the orbital solutions of the respective target systems. Knowledge of the companion masses enabled us to further characterise them in terms of their ages, effective temperatures, and radii via the application of evolutionary models. The inferred ages exhibit a distinct bias towards values younger than what is to be expected based on the literature. The results serve as an independent validation of the orbital solutions published in the NSS two-body orbit catalogue and show that the combination of astrometric survey missions and high-angular-resolution direct imaging holds great promise for efficiently increasing the sample of directly imaged companions in the future, especially in the light of Gaia's upcoming DR4 and the advent of GRAVITY+. △ Less

Submitted 24 June, 2024; v1 submitted 19 March, 2024; originally announced March 2024.

Comments: Accepted for publication in Astronomy and Astrophysics

Journal ref: A&A 688, A44 (2024)

A catalogue of dual-field interferometric binary calibrators

Authors: M. Nowak, S. Lacour, R. Abuter, A. Amorim, R. Asensio-Torres, W. O. Balmer, M. Benisty, J. -P. Berger, H. Beust, S. Blunt, A. Boccaletti, M. Bonnefoy, H. Bonnet, M. S. Bordoni, G. Bourdarot, W. Brandner, F. Cantalloube, B. Charnay, G. Chauvin, A. Chavez, E. Choquet, V. Christiaens, Y. Clénet, V. Coudé du Foresto, A. Cridland , et al. (75 additional authors not shown)

Abstract: Dual-field interferometric observations with VLTI/GRAVITY sometimes require the use of a "binary calibrator", a binary star whose individual components remain unresolved by the interferometer, with a separation between 400 and 2000 mas for observations with the Units Telescopes (UTs), or 1200 to 3000 mas for the Auxiliary Telescopes (ATs). The separation vector also needs to be predictable to with… ▽ More Dual-field interferometric observations with VLTI/GRAVITY sometimes require the use of a "binary calibrator", a binary star whose individual components remain unresolved by the interferometer, with a separation between 400 and 2000 mas for observations with the Units Telescopes (UTs), or 1200 to 3000 mas for the Auxiliary Telescopes (ATs). The separation vector also needs to be predictable to within 10 mas for proper pointing of the instrument. Up until now, no list of properly vetted calibrators was available for dual-field observations with VLTI/GRAVITY on the UTs. Our objective is to compile such a list, and make it available to the community. We identify a list of candidates from the Washington Double Star (WDS) catalogue, all with appropriate separations and brightness, scattered over the Southern sky. We observe them as part of a dedicated calibration programme, and determine whether these objects are true binaries (excluding higher multiplicities resolved interferometrically but unseen by imaging), and extract measurements of the separation vectors. We combine these new measurements with those available in the WDS to determine updated orbital parameters for all our vetted calibrators. We compile a list of 13 vetted binary calibrators for observations with VLTI/GRAVITY on the UTs, and provide orbital estimates and astrometric predictions for each of them. We show that our list guarantees that there are always at least two binary calibrators at airmass < 2 in the sky over the Paranal observatory, at any point in time. Any Principal Investigator wishing to use the dual-field mode of VLTI/GRAVITY with the UTs can now refer to this list to select an appropriate calibrator. We encourage the use of "whereistheplanet" to predict the astrometry of these calibrators, which seamlessly integrates with "p2Gravity" for VLTI/GRAVITY dual-field observing material preparation. △ Less

Submitted 7 February, 2024; originally announced February 2024.

Comments: 16 pages, 6 figures. Submitted to A&A

Journal ref: A&A 687, A248 (2024)

The GRAVITY young stellar object survey XIII. Tracing the time-variable asymmetric disk structure in the inner AU of the Herbig star HD98922

Authors: GRAVITY Collaboration, V. Ganci, L. Labadie, K. Perraut, A. Wojtczak, J. Kaufhold, M. Benisty, E. Alecian, G. Bourdarot, W. Brandner, A. Caratti o Garatti, C. Dougados, R. Garcia Lopez, J. Sanchez-Bermudez, A. Soulain, A. Amorim, J. -P. Berger, P. Caselli, Y. Clénet, A. Drescher, A. Eckart, F. Eisenhauer, M. Fabricius, H. Feuchtgruber, P. Garcia , et al. (30 additional authors not shown)

Abstract: Temporal variability in the photometric and spectroscopic properties of protoplanetary disks is common in YSO. However, evidence pointing toward changes in their morphology over short timescales has only been found for a few sources, mainly due to a lack of high cadence observations at mas resolution. We combine GRAVITY multi-epoch observations of HD98922 at mas resolution with PIONIER archival da… ▽ More Temporal variability in the photometric and spectroscopic properties of protoplanetary disks is common in YSO. However, evidence pointing toward changes in their morphology over short timescales has only been found for a few sources, mainly due to a lack of high cadence observations at mas resolution. We combine GRAVITY multi-epoch observations of HD98922 at mas resolution with PIONIER archival data covering a total time span of 11 years. We interpret the interferometric visibilities and spectral energy distribution with geometrical models and through radiative transfer techniques. We investigated high-spectral-resolution quantities to obtain information on the properties of the HI BrG-line-emitting region. The observations are best fitted by a model of a crescent-like asymmetric dust feature located at 1 au and accounting for 70% of the NIR emission. The feature has an almost constant magnitude and orbits the central star with a possible sub-Keplerian period of 12 months, although a 9 month period is another, albeit less probable, solution. The radiative transfer models show that the emission originates from a small amount of carbon-rich (25%) silicates, or quantum-heated particles located in a low-density region. Among different possible scenarios, we favor hydrodynamical instabilities in the inner disk that can create a large vortex. The high spectral resolution differential phases in the BrG-line show that the hot-gas component is offset from the star and in some cases is located between the star and the crescent feature. The scale of the emission does not favor magnetospheric accretion as a driving mechanism. The scenario of an asymmetric disk wind or a massive accreting substellar or planetary companion is discussed. With this unique observational data set for HD98922, we reveal morphological variability in the innermost 2 au of its disk region. △ Less

Submitted 31 January, 2024; originally announced January 2024.

Comments: 45 pages, 20 figures, accepted by and to be published in Astronomy & Astrophysics (A&A)

A dynamical measure of the black hole mass in a quasar 11 billion years ago

Authors: R. Abuter, F. Allouche, A. Amorim, C. Bailet, A. Berdeu, J. -P. Berger, P. Berio, A. Bigioli, O. Boebion, M. -L. Bolzer, H. Bonnet, G. Bourdarot, P. Bourget, W. Brandner, Y. Cao, R. Conzelmann, M. Comin, Y. Clénet, B. Courtney-Barrer, R. Davies, D. Defrère, A. Delboulbé, F. Delplancke-Ströbele, R. Dembet, J. Dexter , et al. (102 additional authors not shown)

Abstract: Tight relationships exist in the local universe between the central stellar properties of galaxies and the mass of their supermassive black hole. These suggest galaxies and black holes co-evolve, with the main regulation mechanism being energetic feedback from accretion onto the black hole during its quasar phase. A crucial question is how the relationship between black holes and galaxies evolves… ▽ More Tight relationships exist in the local universe between the central stellar properties of galaxies and the mass of their supermassive black hole. These suggest galaxies and black holes co-evolve, with the main regulation mechanism being energetic feedback from accretion onto the black hole during its quasar phase. A crucial question is how the relationship between black holes and galaxies evolves with time; a key epoch to probe this relationship is at the peaks of star formation and black hole growth 8-12 billion years ago (redshifts 1-3). Here we report a dynamical measurement of the mass of the black hole in a luminous quasar at a redshift of 2, with a look back time of 11 billion years, by spatially resolving the broad line region. We detect a 40 micro-arcsecond (0.31 pc) spatial offset between the red and blue photocenters of the H$α$ line that traces the velocity gradient of a rotating broad line region. The flux and differential phase spectra are well reproduced by a thick, moderately inclined disk of gas clouds within the sphere of influence of a central black hole with a mass of 3.2x10$^{8}$ solar masses. Molecular gas data reveal a dynamical mass for the host galaxy of 6x10$^{11}$ solar masses, which indicates an under-massive black hole accreting at a super-Eddington rate. This suggests a host galaxy that grew faster than the supermassive black hole, indicating a delay between galaxy and black hole formation for some systems. △ Less

Submitted 25 January, 2024; originally announced January 2024.

Comments: 5 pages Main text, 8 figures, 2 tables, to be published in Nature, under embargo until 29 January 2024 16:00 (London)

The GRAVITY young stellar object survey XII. The hot gas disk component in Herbig Ae/Be stars

Authors: GRAVITY Collaboration, R. Garcia Lopez, A. Natta, R. Fedriani, A. Caratti o Garatti, J. Sanchez-Bermudez, K. Perraut, C. Dougados, Y. -I. Bouarour, J. Bouvier, W. Brandner, P. Garcia, M. Koutoulaki, L. Labadie, H. Linz, E. Al'ecian, M. Benisty, J. -P. Berger, G. Bourdarot, P. Caselli, Y. Clenet, P. T. de Zeeuw, R. Davies, A. Eckart, F. Eisenhauer , et al. (24 additional authors not shown)

Abstract: The region of protoplanetary disks closest to a star (within 1-2\,au) is shaped by a number of different processes, from accretion of the disk material onto the central star to ejection in the form of winds and jets. Optical and near-IR emission lines are potentially good tracers of inner disk processes if very high spatial and/or spectral resolution are achieved. In this paper, we exploit the cap… ▽ More The region of protoplanetary disks closest to a star (within 1-2\,au) is shaped by a number of different processes, from accretion of the disk material onto the central star to ejection in the form of winds and jets. Optical and near-IR emission lines are potentially good tracers of inner disk processes if very high spatial and/or spectral resolution are achieved. In this paper, we exploit the capabilities of the VLTI-GRAVITY near-IR interferometer to determine the location and kinematics of the hydrogen emission line Bracket gamma. We present VLTI-GRAVITY observations of the Bracket gamma line for a sample of 26 stars of intermediate mass (HAEBE), the largest sample so far analysed with near-IR interferometry. The Bracket gamma line was detected in 17 objects. The emission is very compact (in most cases only marginally resolved), with a size of 10-30R* (1-5 mas). About half of the total flux comes from even smaller regions, which are unresolved in our data. For eight objects, it was possible to determine the position angle (PA) of the line-emitting region, which is generally in agreement with that of the inner-dusty disk emitting the K-band continuum. The position-velocity pattern of the Bracket gamma line-emitting region of the sampled objects is roughly consistent with Keplerian rotation. The exception is HD~45677, which shows more extended emission and more complex kinematics. The most likely scenario for the Bracket gamma origin is that the emission comes from an MHD wind launched very close to the central star, in a region well within the dust sublimation radius. An origin in the bound gas layer at the disk surface cannot be ruled out, while accreting matter provides only a minor fraction of the total flux. These results show the potential of near-IR spectro-interferometry to study line emission in young stellar objects. △ Less

Submitted 15 January, 2024; originally announced January 2024.

Comments: A&A accepted

The GRAVITY young stellar object survey: XI. Imaging the hot gas emission around the Herbig Ae star HD 58647

Authors: Y. -I. Bouarour, R. Garcia Lopez, J. Sanchez-Bermudez, A. Caratti o Garatti, K. Perraut, N. Aimar, A. Amorim, J. -P. Berger, G. Bourdarot, W. Brandner, Y. Clénet, P. T. de Zeeuw, C. Dougados, A. Drescher, A. Eckart, F. Eisenhauer, M. Flock, P. Garcia, E. Gendron, R. Genzel, S. Gillessen, S. Grant, G. Heißel, Th. Henning, L. Jocou , et al. (23 additional authors not shown)

Abstract: We aim to investigate the origin of the HI Br$γ$ emission in young stars by using GRAVITY to image the innermost region of circumstellar disks, where important physical processes such as accretion and winds occur. With high spectral and angular resolution, we focus on studying the continuum and the HI Br$γ$-emitting area of the Herbig star HD58647. Using VLTI-GRAVITY, we conducted observations of… ▽ More We aim to investigate the origin of the HI Br$γ$ emission in young stars by using GRAVITY to image the innermost region of circumstellar disks, where important physical processes such as accretion and winds occur. With high spectral and angular resolution, we focus on studying the continuum and the HI Br$γ$-emitting area of the Herbig star HD58647. Using VLTI-GRAVITY, we conducted observations of HD58647 with both high spectral and high angular resolution. Thanks to the extensive $uv$ coverage, we were able to obtain detailed images of the circumstellar environment at a sub-au scale, specifically capturing the continuum and the Br$γ$-emitting region. Through the analysis of velocity-dispersed images and photocentre shifts, we were able to investigate the kinematics of the HI Br$γ$-emitting region. The recovered continuum images show extended emission where the disk major axis is oriented along a position angle of 14\degr. The size of the continuum emission at 5-sigma levels is $\sim$ 1.5 times more extended than the sizes reported from geometrical fitting (3.69 mas $\pm$ 0.02 mas). This result supports the existence of dust particles close to the stellar surface, screened from the stellar radiation by an optically thick gaseous disk. Moreover, for the first time with GRAVITY, the hot gas component of HD58647 traced by the Br$γ$ ,has been imaged. This allowed us to constrain the size of the Br$γ$-emitting region and study the kinematics of the hot gas; we find its velocity field to be roughly consistent with gas that obeys Keplerian motion. The velocity-dispersed images show that the size of the hot gas emission is from a more compact region than the continuum (2.3 mas $\pm$ 0.2 mas). Finally, the line phases show that the emission is not entirely consistent with Keplerian rotation, hinting at a more complex structure in the hot gaseous disk. △ Less

Submitted 14 December, 2023; originally announced December 2023.

Comments: accepted in A&A on 20/11/2023

VLTI/GRAVITY Provides Evidence the Young, Substellar Companion HD 136164 Ab formed like a "Failed Star"

Authors: William O. Balmer, L. Pueyo, S. Lacour, J. J. Wang, T. Stolker, J. Kammerer, N. Pourré, M. Nowak, E. Rickman, S. Blunt, A. Sivaramakrishnan, D. Sing, K. Wagner, G. -D. Marleau, A. -M. Lagrange, R. Abuter, A. Amorim, R. Asensio-Torres, J. -P. Berger, H. Beust, A. Boccaletti, A. Bohn, M. Bonnefoy, H. Bonnet, M. S. Bordoni , et al. (71 additional authors not shown)

Abstract: Young, low-mass Brown Dwarfs orbiting early-type stars, with low mass ratios ($q\lesssim0.01$), appear intrinsically rare and present a formation dilemma: could a handful of these objects be the highest mass outcomes of ``planetary" formation channels (bottom up within a protoplanetary disk), or are they more representative of the lowest mass ``failed binaries" (formed via disk fragmentation, or c… ▽ More Young, low-mass Brown Dwarfs orbiting early-type stars, with low mass ratios ($q\lesssim0.01$), appear intrinsically rare and present a formation dilemma: could a handful of these objects be the highest mass outcomes of ``planetary" formation channels (bottom up within a protoplanetary disk), or are they more representative of the lowest mass ``failed binaries" (formed via disk fragmentation, or core fragmentation)? Additionally, their orbits can yield model-independent dynamical masses, and when paired with wide wavelength coverage and accurate system age estimates, can constrain evolutionary models in a regime where the models have a wide dispersion depending on initial conditions. We present new interferometric observations of the $16\,\mathrm{Myr}$ substellar companion HD~136164~Ab (HIP~75056~Ab) with VLTI/GRAVITY and an updated orbit fit including proper motion measurements from the Hipparcos-Gaia Catalogue of Accelerations. We estimate a dynamical mass of $35\pm10\,\mathrm{M_J}$ ($q\sim0.02$), making HD~136164~Ab the youngest substellar companion with a dynamical mass estimate. The new mass and newly constrained orbital eccentricity ($e=0.44\pm0.03$) and separation ($22.5\pm1\,\mathrm{au}$) could indicate that the companion formed via the low-mass tail of the Initial Mass Function. Our atmospheric fit to the \texttt{SPHINX} M-dwarf model grid suggests a sub-solar C/O ratio of $0.45$, and $3\times$ solar metallicity, which could indicate formation in the circumstellar disk via disk fragmentation. Either way, the revised mass estimate likely excludes ``bottom-up" formation via core accretion in the circumstellar disk. HD~136164~Ab joins a select group of young substellar objects with dynamical mass estimates; epoch astrometry from future \textit{Gaia} data releases will constrain the dynamical mass of this crucial object further. △ Less

Submitted 13 December, 2023; originally announced December 2023.

Comments: Accepted for publication in The Astronomical Journal. 9 figures, 3 tables

The large-scale magnetic field of the M dwarf double-line spectroscopic binary FK Aqr

Authors: S. Tsvetkova, J. Morin, C. P. Folsom, J. -B. Le Bouquin, E. Alecian, S. Bellotti, G. Hussain, O. Kochukhov, S. C. Marsden, C. Neiner, P. Petit, G. A. Wade, the BinaMIcS collaboration

Abstract: This work is part of the BinaMIcS project, the aim of which is to understand the interaction between binarity and magnetism in close binary systems. All the studied spectroscopic binaries targeted by the BinaMIcS project encompass hot massive and intermediate-mass stars on the main sequence, as well as cool stars over a wide range of evolutionary stages. The present paper focuses on the binary sys… ▽ More This work is part of the BinaMIcS project, the aim of which is to understand the interaction between binarity and magnetism in close binary systems. All the studied spectroscopic binaries targeted by the BinaMIcS project encompass hot massive and intermediate-mass stars on the main sequence, as well as cool stars over a wide range of evolutionary stages. The present paper focuses on the binary system FK Aqr, which is composed of two early M dwarfs. Both stars are already known to be magnetically active based on their light curves and detected flare activity. In addition, the two components have large convective envelopes with masses just above the fully convective limit, making the system an ideal target for studying effect of binarity on stellar dynamos. We use spectropolarimetric observations obtained with ESPaDOnS at CFHT in September 2014. Mean Stokes I and V line profiles are extracted using the least-squares deconvolution (LSD) method. The radial velocities of the two components are measured from the LSD Stokes I profiles and are combined with interferometric measurements in order to constrain the orbital parameters of the system. The longitudinal magnetic fields Bl and chromospheric activity indicators are measured from the LSD mean line profiles. The rotational modulation of the Stokes V profiles is used to reconstruct the surface magnetic field structures of both stars via the Zeeman Doppler imaging (ZDI) inversion technique. Maps of the surface magnetic field structures of both components of FK Aqr are presented for the first time. Our study shows that both components host similar large-scale magnetic fields of moderate intensity (Bmean ~ 0.25 kG); both are predominantly poloidal and feature a strong axisymmetric dipolar component. (abridged) △ Less

Submitted 7 December, 2023; originally announced December 2023.

Comments: 19 pages, 6 figures, 6 tables + appendices, accepted for publication in A&A, in press

Using the motion of S2 to constrain vector clouds around SgrA*

Authors: GRAVITY Collaboration, A. Foschi, R. Abuter, K. Abd El Dayem, N. Aimar, P. Amaro Seoane, A. Amorim, J. P. Berger, H. Bonnet, G. Bourdarot, W. Brandner, R. Davies, P. T. de Zeeuw, D. Defrère, J. Dexter, A. Drescher, A. Eckart, F. Eisenhauer, N. M. Förster Schreiber, P. J. V. Garcia, R. Genzel, S. Gillessen, T. Gomes, X. Haubois, G. Heißel , et al. (31 additional authors not shown)

Abstract: The dark compact object at the centre of the Milky Way is well established to be a supermassive black hole with mass $M_{\bullet} \sim 4.3 \cdot 10^6 \, M_{\odot}$, but the nature of its environment is still under debate. In this work, we used astrometric and spectroscopic measurements of the motion of the star S2, one of the closest stars to the massive black hole, to determine an upper limit on… ▽ More The dark compact object at the centre of the Milky Way is well established to be a supermassive black hole with mass $M_{\bullet} \sim 4.3 \cdot 10^6 \, M_{\odot}$, but the nature of its environment is still under debate. In this work, we used astrometric and spectroscopic measurements of the motion of the star S2, one of the closest stars to the massive black hole, to determine an upper limit on an extended mass composed of a massive vector field around Sagittarius A*. For a vector with effective mass $10^{-19} \, \rm eV \lesssim m_s \lesssim 10^{-18} \, \rm eV$, our Markov Chain Monte Carlo analysis shows no evidence for such a cloud, placing an upper bound $M_{\rm cloud} \lesssim 0.1\% M_{\bullet}$ at $3σ$ confidence level. We show that dynamical friction exerted by the medium on S2 motion plays no role in the analysis performed in this and previous works, and can be neglected thus. △ Less

Submitted 8 February, 2024; v1 submitted 5 December, 2023; originally announced December 2023.

Comments: 9 pages, 5 figures, accepted to MNRAS

Polarization analysis of the VLTI and GRAVITY

Authors: GRAVITY Collaboration, F. Widmann, X. Haubois N. Schuhler, O. Pfuhl, F. Eisenhauer, S. Gillessen, N. Aimar, A. Amorim, M. Bauböck, J. B. Berger, H. Bonnet, G. Bourdarot, W. Brandner, Y. Clénet, R. Davies, P. T. de Zeeuw, J. Dexter, A. Drescher, A. Eckart, H. Feuchtgruber, N. M. Förster Schreiber, P. Garcia, E. Gendron, R. Genzel, M. Hartl , et al. (37 additional authors not shown)

Abstract: The goal of this work is to characterize the polarization effects of the VLTI and GRAVITY. This is needed to calibrate polarimetric observations with GRAVITY for instrumental effects and to understand the systematic error introduced to the astrometry due to birefringence when observing targets with a significant intrinsic polarization. By combining a model of the VLTI light path and its mirrors an… ▽ More The goal of this work is to characterize the polarization effects of the VLTI and GRAVITY. This is needed to calibrate polarimetric observations with GRAVITY for instrumental effects and to understand the systematic error introduced to the astrometry due to birefringence when observing targets with a significant intrinsic polarization. By combining a model of the VLTI light path and its mirrors and dedicated experimental data, we construct a full polarization model of the VLTI UTs and the GRAVITY instrument. We first characterize all telescopes together to construct a UT calibration model for polarized targets. We then expand the model to include the differential birefringence. With this, we can constrain the systematic errors for highly polarized targets. Together with this paper, we publish a standalone Python package to calibrate the instrumental effects on polarimetric observations. This enables the community to use GRAVITY to observe targets in a polarimetric observing mode. We demonstrate the calibration model with the galactic center star IRS 16C. For this source, we can constrain the polarization degree to within 0.4 % and the polarization angle within 5 deg while being consistent with the literature. Furthermore, we show that there is no significant contrast loss, even if the science and fringe-tracker targets have significantly different polarization, and we determine that the phase error in such an observation is smaller than 1 deg, corresponding to an astrometric error of 10 μas. With this work, we enable the use of the polarimetric mode with GRAVITY/UTs and outline the steps necessary to observe and calibrate polarized targets. We demonstrate that it is possible to measure the intrinsic polarization of astrophysical sources with high precision and that polarization effects do not limit astrometric observations of polarized targets. △ Less

Submitted 6 November, 2023; originally announced November 2023.

Comments: Accepted by A&A

First VLTI/GRAVITY Observations of HIP 65426 b: Evidence for a Low or Moderate Orbital Eccentricity

Authors: S. Blunt, W. O. Balmer, J. J. Wang, S. Lacour, S. Petrus, G. Bourdarot, J. Kammerer, N. Pourré, E. Rickman, J. Shangguan, T. Winterhalder, R. Abuter, A. Amorim, R. Asensio-Torres, M. Benisty, J. -P. Berger, H. Beust, A. Boccaletti, A. Bohn, M. Bonnefoy, H. Bonnet, W. Brandner, F. Cantalloube, P. Caselli, B. Charnay , et al. (73 additional authors not shown)

Abstract: Giant exoplanets have been directly imaged over orders of magnitude of orbital separations, prompting theoretical and observational investigations of their formation pathways. In this paper, we present new VLTI/GRAVITY astrometric data of HIP 65426 b, a cold, giant exoplanet which is a particular challenge for most formation theories at a projected separation of 92 au from its primary. Leveraging… ▽ More Giant exoplanets have been directly imaged over orders of magnitude of orbital separations, prompting theoretical and observational investigations of their formation pathways. In this paper, we present new VLTI/GRAVITY astrometric data of HIP 65426 b, a cold, giant exoplanet which is a particular challenge for most formation theories at a projected separation of 92 au from its primary. Leveraging GRAVITY's astrometric precision, we present an updated eccentricity posterior that disfavors large eccentricities. The eccentricity posterior is still prior-dependent, and we extensively interpret and discuss the limits of the posterior constraints presented here. We also perform updated spectral comparisons with self-consistent forward-modeled spectra, finding a best fit ExoREM model with solar metallicity and C/O=0.6. An important caveat is that it is difficult to estimate robust errors on these values, which are subject to interpolation errors as well as potentially missing model physics. Taken together, the orbital and atmospheric constraints paint a preliminary picture of formation inconsistent with scattering after disk dispersal. Further work is needed to validate this interpretation. Analysis code used to perform this work is available at https://github.com/sblunt/hip65426. △ Less

Submitted 6 October, 2023; v1 submitted 29 September, 2023; originally announced October 2023.

Comments: 28 pages, 14 figures. Revised and resubmitted to AJ

VLTI/GRAVITY Observations and Characterization of the Brown Dwarf Companion HD 72946 B

Authors: W. O. Balmer, L. Pueyo, T. Stolker, H. Reggiani, S. Lacour, A. -L. Maire, P. Mollière, M. Nowak, D. Sing, N. Pourré, S. Blunt, J. J. Wang, E. Rickman, Th. Henning, K. Ward-Duong, R. Abuter, A. Amorim, R. Asensio-Torres, M. Benisty, J. -P. Berger, H. Beust, A. Boccaletti, A. Bohn, M. Bonnefoy, H. Bonnet , et al. (74 additional authors not shown)

Abstract: Tension remains between the observed and modeled properties of substellar objects, but objects in binary orbits, with known dynamical masses can provide a way forward. HD 72946 B is a recently imaged brown dwarf companion to the nearby, solar type star. We achieve $\sim100~μ\mathrm{as}$ relative astrometry of HD 72946 B in the K-band using VLTI/GRAVITY, unprecedented for a benchmark brown dwarf. W… ▽ More Tension remains between the observed and modeled properties of substellar objects, but objects in binary orbits, with known dynamical masses can provide a way forward. HD 72946 B is a recently imaged brown dwarf companion to the nearby, solar type star. We achieve $\sim100~μ\mathrm{as}$ relative astrometry of HD 72946 B in the K-band using VLTI/GRAVITY, unprecedented for a benchmark brown dwarf. We fit an ensemble of measurements of the orbit using orbitize! and derive a strong dynamical mass constraint $\mathrm{M_B}=69.5\pm0.5~\mathrm{M_{Jup}}$ assuming a strong prior on the host star mass $\mathrm{M_A}=0.97\pm0.01~\mathrm{M_\odot}$ from an updated stellar analysis. We fit the spectrum of the companion to a grid of self-consistent BT-Settl-CIFIST model atmospheres, and perform atmospheric retrievals using petitRADTRANS. A dynamical mass prior only marginally influences the sampled distribution on effective temperature, but has a large influence on the surface gravity and radius, as expected. The dynamical mass alone does not strongly influence retrieved pressure-temperature or cloud parameters within our current retrieval setup. Independent of cloud prescription and prior assumptions, we find agreement within $\pm2\,σ$ between the C/O ratio of the host ($0.52\pm0.05)$ and brown dwarf ($0.43$ to $0.63$), as expected from a molecular cloud collapse formation scenario, but our retrieved metallicities are implausibly high ($0.6-0.8$) in light of an excellent agreement of the data with the solar abundance model grid. Future work on our retrieval framework will seek to resolve this tension. Additional study of low surface-gravity objects is necessary to assess the influence of a dynamical mass prior on atmospheric analysis. △ Less

Submitted 15 September, 2023; v1 submitted 8 September, 2023; originally announced September 2023.

Comments: Accepted for publication in the Astrophysical Journal. 32 pages, 14 figures, 7 tables. v2&3 correct errors in co-author's affiliations, figure rendering, and some grant acknowledgements

Polarimetry and Astrometry of NIR Flares as Event Horizon Scale, Dynamical Probes for the Mass of Sgr A*

Authors: The GRAVITY Collaboration, R. Abuter, N. Aimar, P. Amaro Seoane, A. Amorim, M. Bauböck, J. P. Berger, H. Bonnet, G. Bourdarot, W. Brandner, V. Cardoso, Y. Clénet, R. Davies, P. T. de Zeeuw, J. Dexter, A. Drescher, A. Eckart, F. Eisenhauer, H. Feuchtgruber, G. Finger, N. M. Förster Schreiber, A. Foschi, P. Garcia, F. Gao, Z. Gelles , et al. (44 additional authors not shown)

Abstract: We present new astrometric and polarimetric observations of flares from Sgr A* obtained with GRAVITY, the near-infrared interferometer at ESO's Very Large Telescope Interferometer (VLTI), bringing the total sample of well-covered astrometric flares to four and polarimetric ones to six, where we have for two flares good coverage in both domains. All astrometric flares show clockwise motion in the p… ▽ More We present new astrometric and polarimetric observations of flares from Sgr A* obtained with GRAVITY, the near-infrared interferometer at ESO's Very Large Telescope Interferometer (VLTI), bringing the total sample of well-covered astrometric flares to four and polarimetric ones to six, where we have for two flares good coverage in both domains. All astrometric flares show clockwise motion in the plane of the sky with a period of around an hour, and the polarization vector rotates by one full loop in the same time. Given the apparent similarities of the flares, we present a common fit, taking into account the absence of strong Doppler boosting peaks in the light curves and the EHT-measured geometry. Our results are consistent with and significantly strengthen our model from 2018: We find that a) the combination of polarization period and measured flare radius of around nine gravitational radii ($9 R_g \approx 1.5 R_{ISCO}$, innermost stable circular orbit) is consistent with Keplerian orbital motion of hot spots in the innermost accretion zone. The mass inside the flares' radius is consistent with the $4.297 \times 10^6 \; \text{M}_\odot$ measured from stellar orbits at several thousand $R_g$. This finding and the diameter of the millimeter shadow of Sgr A* thus support a single black hole model. Further, b) the magnetic field configuration is predominantly poloidal (vertical), and the flares' orbital plane has a moderate inclination with respect to the plane of the sky, as shown by the non-detection of Doppler-boosting and the fact that we observe one polarization loop per astrometric loop. Moreover, c) both the position angle on sky and the required magnetic field strength suggest that the accretion flow is fueled and controlled by the winds of the massive, young stars of the clockwise stellar disk 1-5 arcsec from Sgr A*, in agreement with recent simulations. △ Less

Submitted 31 August, 2023; v1 submitted 21 July, 2023; originally announced July 2023.

Comments: 10 pages, 12 figures. Accepted for publication in A&A

Journal ref: A&A 677, L10 (2023)

The Great Dimming of the hypergiant star RW Cephei: CHARA Array images and spectral analysis

Authors: N. Anugu, F. Baron, D. R. Gies, C. Lanthermann, G. H. Schaefer, K. A. Shepard, T. ten Brummelaar, J. D. Monnier, S. Kraus, J. -B. Le Bouquin, C. L. Davies, J. Ennis, T. Gardner, A. Labdon, R. M. Roettenbacher, B. R. Setterholm, W. Vollmann, C. Sigismondi

Abstract: The cool hypergiant star RW Cephei is currently in a deep photometric minimum that began several years ago. This event bears a strong similarity to the Great Dimming of the red supergiant Betelgeuse that occurred in 2019-2020. We present the first resolved images of RW Cephei that we obtained with the CHARA Array interferometer. The angular diameter and Gaia distance estimates indicate a stellar r… ▽ More The cool hypergiant star RW Cephei is currently in a deep photometric minimum that began several years ago. This event bears a strong similarity to the Great Dimming of the red supergiant Betelgeuse that occurred in 2019-2020. We present the first resolved images of RW Cephei that we obtained with the CHARA Array interferometer. The angular diameter and Gaia distance estimates indicate a stellar radius of 900 - 1760 R_sun which makes RW Cep one of the largest stars known in the Milky Way. The reconstructed, near-infrared images show a striking asymmetry in the disk illumination with a bright patch offset from center and a darker zone to the west. The imaging results depend on assumptions made about the extended flux, and we present two cases with and without allowing extended emission. We also present a recent near-infrared spectrum of RW Cep that demonstrates that the fading is much larger at visual wavelengths compared to that at near-infrared wavelengths as expected for extinction by dust. We suggest that the star's dimming is the result of a recent surface mass ejection event that created a dust cloud that now partially blocks the stellar photosphere. △ Less

Submitted 10 July, 2023; originally announced July 2023.

Comments: 20 pages, accepted for AJ

Where intermediate-mass black holes could hide in the Galactic Centre: A full parameter study with the S2 orbit

Authors: The GRAVITY Collaboration, O. Straub, M. Bauböck, R. Abuter, N. Aimar, P. Amaro Seoane, A. Amorim, J. P. Berger, H. Bonnet, G. Bourdarot, W. Brandner, V. Cardoso, Y. Clénet, Y. Dallilar, R. Davies, P. T. de Zeeuw, J. Dexter, A. Drescher, F. Eisenhauer, N. M. Förster Schreiber, A. Foschi, P. Garcia, F. Gao, E. Gendron, R. Genzel , et al. (37 additional authors not shown)

Abstract: In the Milky Way the central massive black hole, SgrA*, coexists with a compact nuclear star cluster that contains a sub-parsec concentration of fast-moving young stars called S-stars. Their location and age are not easily explained by current star formation models, and in several scenarios the presence of an intermediate-mass black hole (IMBH) has been invoked. We use GRAVITY astrometric and SINF… ▽ More In the Milky Way the central massive black hole, SgrA*, coexists with a compact nuclear star cluster that contains a sub-parsec concentration of fast-moving young stars called S-stars. Their location and age are not easily explained by current star formation models, and in several scenarios the presence of an intermediate-mass black hole (IMBH) has been invoked. We use GRAVITY astrometric and SINFONI, KECK, and GNIRS spectroscopic data of S2 to investigate whether a second massive object could be present deep in the Galactic Centre (GC) in the form of an IMBH binary companion to SgrA*. To solve the three-body problem, we used a post-Newtonian framework and consider two types of settings: (i) a hierarchical set-up where the star S2 orbits the SgrA* - IMBH binary and (ii) a non-hierarchical set-up where the IMBH trajectory lies outside the S2 orbit. In both cases we explore the full 20-dimensional parameter space by employing a Bayesian dynamic nested sampling method. For the hierarchical case we find: IMBH masses > 2000 Msun on orbits with smaller semi-major axes than S2 are largely excluded. For the non-hierarchical case the parameter space contains several pockets of valid IMBH solutions. However, a closer analysis of their impact on the resident stars reveals that IMBHs on semi-major axes larger than S2 tend to disrupt the S-star cluster in less than a million years. This makes the existence of an IMBH among the S-stars highly unlikely. The current S2 data do not formally require the presence of an IMBH. If an IMBH hides in the GC, it has to be either a low-mass IMBH inside the S2 orbit that moves on a short and significantly inclined trajectory or an IMBH with a semi-major axis >1". We provide the parameter maps of valid IMBH solutions in the GC and discuss the general structure of our results. (abridged) △ Less

Submitted 13 July, 2023; v1 submitted 7 March, 2023; originally announced March 2023.

Comments: 11 pages, 6 figures, published in A $\&$ A

Journal ref: A&A 672, A63 (2023)

The GRAVITY Young Stellar Object survey -- IX. Spatially resolved kinematics of hot hydrogen gas in the star/disk interaction region of T Tauri stars

Authors: GRAVITY Collaboration, J. A. Wojtczak, L. Labadie, K. Perraut, B. Tessore, A. Soulain, V. Ganci, J. Bouvier, C. Dougados, E. Alécian, H. Nowacki, G. Cozzo, W. Brandner, A. Caratti o Garatti, P. Garcia, R. Garcia Lopez, J. Sanchez-Bermudez, A. Amorim, M. Benisty, J. -P. Berger, G. Bourdarot, P. Caselli, Y. Clénet, P. T. de Zeeuw, R. Davies , et al. (36 additional authors not shown)

Abstract: Aims: We aim to spatially and spectrally resolve the Br-gamma hydrogen emission line with the methods of interferometry in order to examine the kinematics of the hydrogen gas emission region in the inner accretion disk of a sample of solar-like young stellar objects. The goal is to identify trends and categories among the sources of our sample and to discuss whether or not they can be tied to diff… ▽ More Aims: We aim to spatially and spectrally resolve the Br-gamma hydrogen emission line with the methods of interferometry in order to examine the kinematics of the hydrogen gas emission region in the inner accretion disk of a sample of solar-like young stellar objects. The goal is to identify trends and categories among the sources of our sample and to discuss whether or not they can be tied to different origin mechanisms associated with Br-gamma emission in T Tauri stars, chiefly and most prominently magnetospheric accretion. Methods: We observed a sample of seven T Tauri stars for the first time with VLTI GRAVITY, recording spectra and spectrally dispersed interferometric quantities across the Br-gamma line in the NIR K-band. We use them to extract the size of the Br-gamma emission region and the photocenter shifts. To assist in the interpretation, we also make use of radiative transfer models of magnetospheric accretion to establish a baseline of expected interferometric signatures if accretion is the primary driver of Br-gamma emission. Results: From among our sample, we find that five of the seven T~Tauri stars show an emission region with a half-flux radius in the range broadly expected for magnetospheric truncation. Two of the five objects also show Br-gamma emission primarily originating from within the corotation radius, while two other objects exhibit extended emission on a scale beyond 10 R$_*$, one of them even beyond the K~band continuum half-flux radius of 11.3 R$_*$. Conclusions: We find strong evidence to suggest that for the two weakest accretors in the sample, magnetospheric accretion is the primary driver of Br-gamma radiation. The results for the remaining sources imply either partial or strong contributions coming from spatially extended emission components in the form of outflows, such as stellar or disk winds. △ Less

Submitted 23 November, 2022; v1 submitted 24 October, 2022; originally announced October 2022.

Comments: Version corresponds to the one in A&A production. The author list has been amended with "GRAVITY Collaboration" as first entry in keeping with the convention established for papers published by the GRAVITY consortium in recent years

Journal ref: A&A 669, A59 (2023)

Dynamical masses of the primary Be star and the secondary sdB star in the single-lined binary kappa Dra (B6 IIIe)

Authors: R. Klement, D. Baade, Th. Rivinius, D. R. Gies, L. Wang, J. Labadie-Bartz, P. Ticiani Dos Santos, J. D. Monnier, A. C. Carciofi, A. Mérand, N. Anugu, G. H. Schaefer, J. -B. Le Bouquin, C. L. Davies, J. Ennis, T. Gardner, S. Kraus, B. R. Setterholm, A. Labdon

Abstract: Because many classical Be stars may owe their nature to mass and angular-momentum transfer in a close binary, the present masses, temperatures, and radii of their components are of high interest for comparison to stellar evolution models. Kappa Dra is a 61.5-day single-lined binary with a B6 IIIe primary. With the CHARA Array instruments MIRC/MIRC-X and MYSTIC, we detected the secondary at (approx… ▽ More Because many classical Be stars may owe their nature to mass and angular-momentum transfer in a close binary, the present masses, temperatures, and radii of their components are of high interest for comparison to stellar evolution models. Kappa Dra is a 61.5-day single-lined binary with a B6 IIIe primary. With the CHARA Array instruments MIRC/MIRC-X and MYSTIC, we detected the secondary at (approximately photospheric) flux ratios of 1.49 +- 0.10% and 1.63 +- 0.09% in the H and K band, respectively. From a large and diverse optical spectroscopic database only the radial velocity curve of the Be star could be extracted. However, employing the parallaxes from Hipparcos and Gaia, which agree within their nominal 1-sigma errors, we could derive the total mass and found component masses of 3.65 +- 0.48 Msun and 0.426 +- 0.043 Msun for the Be star and the companion, respectively. Previous cross-correlation of the observed far-UV spectrum with sdO spectral model templates had not detected a companion belonging to the hot O-type subdwarf (sdO) population known from ~20 earlier-type Be stars. Guided by our full 3D orbital solution, we found a strong cross-correlation signal for a stripped subdwarf B-type companion (far-UV flux ratio of 2.3 +- 0.5%), enabling the first firm characterization of such a star, and making kappa Dra the first mid- to late-type Be star with a directly-observed subdwarf companion. △ Less

Submitted 6 October, 2022; originally announced October 2022.

Comments: Accepted to ApJ. Figure sets available on request

GRAVITY+ Wide: Towards hundreds of z $\sim$ 2 AGN

Authors: A. Drescher, M. Fabricius, T. Shimizu, J. Woillez, P. Bourget, F. Widmann, J. Shangguan, C. Straubmeier, M. Horrobin, N. Schuhler, F. Eisenhauer, F. Gonté, S. Gillessen, T. Ott, G. Perrin, T. Paumard, W. Brandner, L. Kreidberg, K. Perraut, J. -B. Le Bouquin, P. Garcia, S. Hönig, D. Defrère, G. Bourdarot, H. Feuchtgruber , et al. (12 additional authors not shown)

Abstract: As part of the GRAVITY$^{+}$ project, the near-infrared beam combiner GRAVITY and the VLTI are currently undergoing a series of significant upgrades to further improve the performance and sky coverage. The instrumental changes will be transformational, and for instance uniquely position GRAVITY to observe the broad line region of hundreds of Active Galactic Nuclei (AGN) at a redshift of two and hi… ▽ More As part of the GRAVITY$^{+}$ project, the near-infrared beam combiner GRAVITY and the VLTI are currently undergoing a series of significant upgrades to further improve the performance and sky coverage. The instrumental changes will be transformational, and for instance uniquely position GRAVITY to observe the broad line region of hundreds of Active Galactic Nuclei (AGN) at a redshift of two and higher. The increased sky coverage is achieved by enlarging the maximum angular separation between the celestial science object (SC) and the off-axis fringe tracking (FT) star from currently 2 arcseconds (arcsec) up to unprecedented 30 arcsec, limited by the atmospheric conditions. This was successfully demonstrated at the VLTI for the first time. △ Less

Submitted 23 September, 2022; originally announced September 2022.

Comments: Proceeding of SPIE Astronomical Telescopes + Instrumentation 2022

The binary system of the spinning-top Be star Achernar

Authors: P. Kervella, S. Borgniet, A. Domiciano de Souza, A. Mérand, A. Gallenne, Th. Rivinius, S. Lacour, A. Carciofi, D. Moser Faes, J. -B. Le Bouquin, M. Taormina, B. Pilecki, J. -Ph. Berger, Ph. Bendjoya, R. Klement, F. Millour, E. Janot-Pacheco, A. Spang, F. Vakili

Abstract: Achernar, the closest and brightest classical Be star, presents rotational flattening, gravity darkening, occasional emission lines due to a gaseous disk, and an extended polar wind. It is also a member of a close binary system with an early A-type dwarf companion. We aim to determine the orbital parameters of the Achernar system and to estimate the physical properties of the components. We monito… ▽ More Achernar, the closest and brightest classical Be star, presents rotational flattening, gravity darkening, occasional emission lines due to a gaseous disk, and an extended polar wind. It is also a member of a close binary system with an early A-type dwarf companion. We aim to determine the orbital parameters of the Achernar system and to estimate the physical properties of the components. We monitored the relative position of Achernar B using a broad range of high angular resolution instruments of the VLT/VLTI (VISIR, NACO, SPHERE, AMBER, PIONIER, GRAVITY, and MATISSE) over a period of 13 years (2006-2019). These astrometric observations are complemented with a series of more than 700 optical spectra for the period from 2003 to 2016. We determine that Achernar B orbits the Be star on a seven-year period, eccentric orbit (e = 0.7255 +/- 0.0014) which brings the two stars within 2 au at periastron. The mass of the Be star is found to be mA = 6.0 +/- 0.6 Msun for a secondary mass of mB = 2.0 +/- 0.1 Msun. We find a good agreement of the parameters of Achernar A with the evolutionary model of a critically rotating star of 6.4 Msun at an age of 63 million years. We also identify a resolved comoving low-mass star, which leads us to propose that Achernar is a member of the Tucana-Horologium moving group. Achernar A is presently in a short-lived phase of its evolution following the turn-off, during which its geometrical flattening ratio is the most extreme. Considering the orbital parameters, no significant interaction occurred between the two components, demonstrating that Be stars may form through a direct, single-star evolution path without mass transfer. Since component A will enter the instability strip in a few hundred thousand years, Achernar appears to be a promising progenitor of the Cepheid binary systems. △ Less

Submitted 15 September, 2022; originally announced September 2022.

Comments: 27 pages, Astronomy & Astrophysics, in press. The full set of continuum normalized high resolution spectra of Achernar is available at https://doi.org/10.5281/zenodo.6977303

Journal ref: A&A 667, A111 (2022)

GRAVITY faint: reducing noise sources in GRAVITY$^+$ with a fast metrology attenuation system

Authors: F. Widmann, S. Gillessen, T. Ott, T. Shimizu, F. Eisenhauer, M. Fabricius, J. Woillez, F. Gonté, M. Horrobin, J. Shangguan, S. Yazici, G. Perrin, T. Paumard, W. Brandner, L. Kreidberg, C. Straubmeier, K. Perraut, J. -B. Le Bouquin, P. Garcia, S. Hönig, D. Defrère, G. Bourdarot, A. Drescher, H. Feuchtgruber, R. Genzel , et al. (6 additional authors not shown)

Abstract: With the upgrade from GRAVITY to GRAVITY$^+$ the instrument will evolve into an all-sky interferometer that can observe faint targets, such as high redshift AGN. Observing the faintest targets requires reducing the noise sources in GRAVITY as much as possible. The dominant noise source, especially in the blue part of the spectrum, is the backscattering of the metrology laser light onto the detecto… ▽ More With the upgrade from GRAVITY to GRAVITY$^+$ the instrument will evolve into an all-sky interferometer that can observe faint targets, such as high redshift AGN. Observing the faintest targets requires reducing the noise sources in GRAVITY as much as possible. The dominant noise source, especially in the blue part of the spectrum, is the backscattering of the metrology laser light onto the detector. To reduce this noise we introduce two new metrology modes. With a combination of small hardware changes and software adaptations, we can dim the metrology laser during the observation without losing the phase referencing. For single beam targets, we can even turn off the metrology laser for the maximum SNR on the detector. These changes lead to an SNR improvement of over a factor of two averaged over the whole spectrum and up to a factor of eight in the part of the spectrum currently dominated by laser noise. △ Less

Submitted 12 September, 2022; originally announced September 2022.

Comments: Proceeding of SPIE Astronomical Telescopes + Instrumentation 2022

Combining Hipparcos and Gaia data for the study of binaries: the BINARYS tool

Authors: A. Leclerc, C. Babusiaux, F. Arenou, F. van Leeuwen, M. Bonnefoy, X. Delfosse, T. Forveille, J. -B. Le Bouquin, L. Rodet

Abstract: Orbital motion in binary and planetary systems is the main source of precise stellar and planetary mass measurements, and joint analysis of data from multiple observational methods can both lift degeneracies and improve precision. We set out to measure the masses of individual stars in binary systems using all the information brought by the Hipparcos and Gaia absolute astrometric missions. We pres… ▽ More Orbital motion in binary and planetary systems is the main source of precise stellar and planetary mass measurements, and joint analysis of data from multiple observational methods can both lift degeneracies and improve precision. We set out to measure the masses of individual stars in binary systems using all the information brought by the Hipparcos and Gaia absolute astrometric missions. We present BINARYS, a tool which uses the Hipparcos and Gaia absolute astrometric data and combines it with relative astrometry and/or radial velocity measurements to determine the orbit of a binary system. It rigorously combines the Hipparcos and Gaia data (here EDR3), and it can use the Hipparcos Transit Data as needed for binaries where Hipparcos detect significant flux from the secondary component. It also support the case where Gaia resolved the system, giving an astrometric solution for both components. We determine model-independent individual masses for the first time for three systems: the two mature binaries Gl~494 ($M_1=0.584 \pm 0.003 M_{\odot}$ and $M_2=87 \pm 1 M_{\textrm{Jup}}$) and HIP~88745 ($M_1=0.96 \pm 0.02 M_{\odot}$ and $M_2= 0.60^{+ 0.02 }_{- 0.01 } M_{\odot}$), and the younger AB Dor member GJ~2060 ($M_1=0.60 ^{+ 0.06}_{- 0.05} M_{\odot}$ and $M_2=0.45 ^{+ 0.06}_{- 0.05}M_{\odot}$). The latter provides a rare test of evolutionary model predictions at young ages in the low stellar-mass range and sets a lower age limit of 100~Myr for the moving group. △ Less

Submitted 9 September, 2022; originally announced September 2022.

Journal ref: A&A 672, A82 (2023)

Direct discovery of the inner exoplanet in the HD206893 system. Evidence for deuterium burning in a planetary-mass companion

Authors: S. Hinkley, S. Lacour, G. -D. Marleau, A. M. Lagrange, J. J. Wang, J. Kammerer, A. Cumming, M. Nowak, L. Rodet, T. Stolker, W. -O. Balmer, S. Ray, M. Bonnefoy, P. Mollière, C. Lazzoni, G. Kennedy, C. Mordasini, R. Abuter, S. Aigrain, A. Amorim, R. Asensio-Torres, C. Babusiaux, M. Benisty, J. -P. Berger, H. Beust , et al. (89 additional authors not shown)

Abstract: Long term precise radial velocity (RV) monitoring of the nearby star HD206893, as well as anomalies in the system proper motion, have suggested the presence of an additional, inner companion in the system. Here we describe the results of a multi-epoch search for the companion responsible for this RV drift and proper motion anomaly using the VLTI/GRAVITY instrument. Utilizing information from ongoi… ▽ More Long term precise radial velocity (RV) monitoring of the nearby star HD206893, as well as anomalies in the system proper motion, have suggested the presence of an additional, inner companion in the system. Here we describe the results of a multi-epoch search for the companion responsible for this RV drift and proper motion anomaly using the VLTI/GRAVITY instrument. Utilizing information from ongoing precision RV measurements with the HARPS spectrograph, as well as Gaia host star astrometry, we report a high significance detection of the companion HD206893c over three epochs, with clear evidence for Keplerian orbital motion. Our astrometry with $\sim$50-100 $μ$arcsec precision afforded by GRAVITY allows us to derive a dynamical mass of 12.7$^{+1.2}_{-1.0}$ M$_{\rm Jup}$ and an orbital separation of 3.53$^{+0.08}_{-0.06}$ au for HD206893c. Our fits to the orbits of both companions in the system utilize both Gaia astrometry and RVs to also provide a precise dynamical estimate of the previously uncertain mass of the B component, and therefore derive an age of $155\pm15$ Myr. We find that theoretical atmospheric/evolutionary models incorporating deuterium burning for HD206893c, parameterized by cloudy atmospheres provide a good simultaneous fit to the luminosity of both HD206893B and c. In addition to utilizing long-term RV information, this effort is an early example of a direct imaging discovery of a bona fide exoplanet that was guided in part with Gaia astrometry. Utilizing Gaia astrometry is expected to be one of the primary techniques going forward to identify and characterize additional directly imaged planets. Lastly, this discovery is another example of the power of optical interferometry to directly detect and characterize extrasolar planets where they form at ice-line orbital separations of 2-4\,au. △ Less

Submitted 3 April, 2023; v1 submitted 9 August, 2022; originally announced August 2022.

Comments: Accepted to A&A

A detailed analysis of the Gl 486 planetary system

Authors: J. A. Caballero, E. Gonzalez-Alvarez, M. Brady, T. Trifonov, T. G. Ellis, C. Dorn, C. Cifuentes, K. Molaverdikhani, J. L. Bean, T. Boyajian, E. Rodriguez, J. Sanz-Forcada, M. R. Zapatero Osorio, C. Abia, P. J. Amado, N. Anugu, V. J. S. Bejar, C. L. Davies, S. Dreizler, F. Dubois, J. Ennis, N. Espinoza, C. D. Farrington, A. Garcia Lopez, T. Gardner , et al. (42 additional authors not shown)

Abstract: The Gl 486 system consists of a very nearby, relatively bright, weakly active M3.5 V star at just 8 pc with a warm transiting rocky planet of about 1.3 R_Terra and 3.0 M_Terra that is ideal for both transmission and emission spectroscopy and for testing interior models of telluric planets. To prepare for future studies, we collected light curves of seven new transits observed with the CHEOPS space… ▽ More The Gl 486 system consists of a very nearby, relatively bright, weakly active M3.5 V star at just 8 pc with a warm transiting rocky planet of about 1.3 R_Terra and 3.0 M_Terra that is ideal for both transmission and emission spectroscopy and for testing interior models of telluric planets. To prepare for future studies, we collected light curves of seven new transits observed with the CHEOPS space mission and new radial velocities obtained with MAROON-X/Gemini North and CARMENES/Calar Alto telescopes, together with previously published spectroscopic and photometric data from the two spectrographs and TESS. We also performed interferometric observations with the CHARA Array and new photometric monitoring with a suite of smaller telescopes. From interferometry, we measure a limb-darkened disc angular size of the star Gl 486. Together with a corrected Gaia EDR3 parallax, we obtain a stellar radius. We also measure a stellar rotation period at P_rot ~ 49.9 d, an upper limit to its XUV (5-920 AA) flux with new Hubble/STIS data, and, for the first time, a variety of element abundances (Fe, Mg, Si, V, Sr, Zr, Rb) and C/O ratio. Besides, we impose restrictive constraints on the presence of additional components, either stellar or substellar, in the system. With the input stellar parameters and the radial-velocity and transit data, we determine the radius and mass of the planet Gl 486 b at R_p = 1.343+/0.063 R_Terra and M_p = 3.00+/-0.13 M_Terra. From the planet parameters and the stellar element abundances, we infer the most probable models of planet internal structure and composition, which are consistent with a relatively small metallic core with respect to the Earth, a deep silicate mantle, and a thin volatile upper layer. With all these ingredients, we outline prospects for Gl 486 b atmospheric studies, especially with forthcoming James Webb Space Telescope observations (abridged). △ Less

Submitted 20 June, 2022; originally announced June 2022.

Comments: A&A, in press. See https://carmenes.caha.es/ext/pressreleases/GJ486/Exoearth.Video.mp4

Journal ref: A&A 665, A120 (2022)

First Light for GRAVITY Wide: Large Separation Fringe Tracking for the Very Large Telescope Interferometer

Authors: GRAVITY+ Collaboration, :, R. Abuter, F. Allouche, A. Amorim, C. Bailet, M. Bauböck, J. -P. Berger, P. Berio, A. Bigioli, O. Boebion, M. L. Bolzer, H. Bonnet, G. Bourdarot, P. Bourget, W. Brandner, Y. Clénet, B. Courtney-Barrer, Y. Dallilar, R. Davies, D. Defrère, A. Delboulbé, F. Delplancke, R. Dembet, P. T. de Zeeuw , et al. (92 additional authors not shown)

Abstract: GRAVITY+ is the upgrade of GRAVITY and the Very Large Telescope Interferometer (VLTI) with wide-separation fringe tracking, new adaptive optics, and laser guide stars on all four 8~m Unit Telescopes (UTs), for ever fainter, all-sky, high contrast, milliarcsecond interferometry. Here we present the design and first results of the first phase of GRAVITY+, called GRAVITY Wide. GRAVITY Wide combines t… ▽ More GRAVITY+ is the upgrade of GRAVITY and the Very Large Telescope Interferometer (VLTI) with wide-separation fringe tracking, new adaptive optics, and laser guide stars on all four 8~m Unit Telescopes (UTs), for ever fainter, all-sky, high contrast, milliarcsecond interferometry. Here we present the design and first results of the first phase of GRAVITY+, called GRAVITY Wide. GRAVITY Wide combines the dual-beam capabilities of the VLTI and the GRAVITY instrument to increase the maximum separation between the science target and the reference star from 2 arcseconds with the 8 m UTs up to several 10 arcseconds, limited only by the Earth's turbulent atmosphere. This increases the sky-coverage of GRAVITY by two orders of magnitude, opening up milliarcsecond resolution observations of faint objects, and in particular the extragalactic sky. The first observations in 2019 - 2022 include first infrared interferometry of two redshift $z\sim2$ quasars, interferometric imaging on the binary system HD 105913A, and repeated observations of multiple star systems in the Orion Trapezium Cluster. We find the coherence loss between the science object and fringe-tracking reference star well described by the turbulence of the Earth's atmosphere. We confirm that the larger apertures of the UTs result in higher visibilities for a given separation due to larger overlap of the projected pupils on sky and give predictions for visibility loss as a function of separation to be used for future planning. △ Less

Submitted 23 August, 2022; v1 submitted 1 June, 2022; originally announced June 2022.

Comments: 15 pages, 12 figures, 5 tables. Accepted by A&A

Journal ref: A&A 665, A75 (2022)

A dusty veil shading Betelgeuse during its Great Dimming

Authors: M. Montargès, E. Cannon, E. Lagadec, A. de Koter, P. Kervella, J. Sanchez-Bermudez, C. Paladini, F. Cantalloube, L. Decin, P. Scicluna, K. Kravchenko, A. K. Dupree, S. Ridgway, M. Wittkowski, N. Anugu, R. Norris, G. Rau, G. Perrin, A. Chiavassa, S. Kraus, J. D. Monnier, F. Millour, J. -B. Le Bouquin, X. Haubois, B. Lopez , et al. (2 additional authors not shown)

Abstract: Red supergiants are the most common final evolutionary stage of stars that have initial masses between 8 and 35 times that of the Sun. During this stage, which lasts roughly 100,000 years1, red supergiants experience substantial mass loss. However, the mechanism for this mass loss is unknown. Mass loss may affect the evolutionary path, collapse and future supernova light curve of a red supergiant,… ▽ More Red supergiants are the most common final evolutionary stage of stars that have initial masses between 8 and 35 times that of the Sun. During this stage, which lasts roughly 100,000 years1, red supergiants experience substantial mass loss. However, the mechanism for this mass loss is unknown. Mass loss may affect the evolutionary path, collapse and future supernova light curve of a red supergiant, and its ultimate fate as either a neutron star or a black hole. From November 2019 to March 2020, Betelgeuse - the second-closest red supergiant to Earth (roughly 220 parsecs, or 724 light years, away) - experienced a historic dimming of its visible brightness. Usually having an apparent magnitude between 0.1 and 1.0, its visual brightness decreased to 1.614 +/- 0.008 magnitudes around 7-13 February 2020 - an event referred to as Betelgeuse's Great Dimming. Here we report high-angular-resolution observations showing that the southern hemisphere of Betelgeuse was ten times darker than usual in the visible spectrum during its Great Dimming. Observations and modelling support a scenario in which a dust clump formed recently in the vicinity of the star, owing to a local temperature decrease in a cool patch that appeared on the photosphere. The directly imaged brightness variations of Betelgeuse evolved on a timescale of weeks. Our findings suggest that a component of mass loss from red supergiants is inhomogeneous, linked to a very contrasted and rapidly changing photosphere △ Less

Submitted 25 January, 2022; originally announced January 2022.

Journal ref: Nature, 2021, Volume 594, Issue 7863, p.365-368

The mass distribution in the Galactic Centre from interferometric astrometry of multiple stellar orbits

Authors: GRAVITY Collaboration, R. Abuter, N. Aimar, A. Amorim, J. Ball, M. Bauböck, J. P. Berger, H. Bonnet, G. Bourdarot, W. Brandner, V. Cardoso, Y. Clénet, Y. Dallilar, R. Davies, P. T. de Zeeuw, J. Dexter, A. Drescher, F. Eisenhauer, N. M. Förster Schreiber, A. Foschi, P. Garcia, F. Gao, E. Gendron, R. Genzel, S. Gillessen , et al. (40 additional authors not shown)

Abstract: The stars orbiting the compact radio source Sgr A* in the Galactic Centre are precision probes of the gravitational field around the closest massive black hole. In addition to adaptive optics assisted astrometry (with NACO / VLT) and spectroscopy (with SINFONI / VLT, NIRC2 / Keck and GNIRS / Gemini) over three decades, since 2016/2017 we have obtained 30-100 mu-as astrometry with the four-telescop… ▽ More The stars orbiting the compact radio source Sgr A* in the Galactic Centre are precision probes of the gravitational field around the closest massive black hole. In addition to adaptive optics assisted astrometry (with NACO / VLT) and spectroscopy (with SINFONI / VLT, NIRC2 / Keck and GNIRS / Gemini) over three decades, since 2016/2017 we have obtained 30-100 mu-as astrometry with the four-telescope interferometric beam combiner GRAVITY / VLTI reaching a sensitivity of mK = 20 when combining data from one night. We present the simultaneous detection of several stars within the diffraction limit of a single telescope, illustrating the power of interferometry. The new data for the stars S2, S29, S38 and S55 yield significant accelerations between March and July 2021, as these stars pass the pericenters of their orbits between 2018 and 2023. This allows for a high-precision determination of the gravitational potential around Sgr A*. Our data are in excellent agreement with general relativity orbits around a single central point mass, M = 4.30 x 10^6 M_sun with a precision of about +-0.25%. We improve the significance of our detection of the Schwarzschild precession in the S2 orbit to 7 sigma. Assuming plausible density profiles, an extended mass component inside S2's apocentre (= 0.23" or 2.4 x 10^4 R_S) must be 3000 M_sun (1 sigma), or 0.1% of M. Adding the enclosed mass determinations from 13 stars orbiting Sgr A* at larger radii, the innermost radius at which the excess mass beyond Sgr A* tentatively is seen is r = 2.5" >= 10x the apocentre of S2. This is in full harmony with the stellar mass distribution (including stellar-mass black holes) obtained from the spatially resolved luminosity function. △ Less

Submitted 14 December, 2021; originally announced December 2021.

Comments: published in A&A

Journal ref: A&A, 2021,

Deep Images of the Galactic Center with GRAVITY

Authors: GRAVITY Collaboration, R. Abuter, N. Aimar, A. Amorim, P. Arras, M. Bauböck, J. P. Berger, H. Bonnet, W. Brandner, G. Bourdarot, V. Cardoso, Y. Clénet, R. Davies, P. T. de Zeeuw, J. Dexter, Y. Dallilar, A. Drescher, F. Eisenhauer, T. Enßlin, N. M. Förster Schreiber, P. Garcia, F. Gao, E. Gendron, R. Genzel, S. Gillessen , et al. (43 additional authors not shown)

Abstract: Stellar orbits at the Galactic Center provide a very clean probe of the gravitational potential of the supermassive black hole. They can be studied with unique precision, beyond the confusion limit of a single telescope, with the near-infrared interferometer GRAVITY. Imaging is essential to search the field for faint, unknown stars on short orbits which potentially could constrain the black hole s… ▽ More Stellar orbits at the Galactic Center provide a very clean probe of the gravitational potential of the supermassive black hole. They can be studied with unique precision, beyond the confusion limit of a single telescope, with the near-infrared interferometer GRAVITY. Imaging is essential to search the field for faint, unknown stars on short orbits which potentially could constrain the black hole spin. Furthermore, it provides the starting point for astrometric fitting to derive highly accurate stellar positions. Here, we present $\mathrm{G^R}$, a new imaging tool specifically designed for Galactic Center observations with GRAVITY. The algorithm is based on a Bayesian interpretation of the imaging problem, formulated in the framework of information field theory and building upon existing works in radio-interferometric imaging. Its application to GRAVITY observations from 2021 yields the deepest images to date of the Galactic Center on scales of a few milliarcseconds. The images reveal the complicated source structure within the central $100\,\mathrm{mas}$ around Sgr A*, where we detected the stars S29 and S55 and confirm S62 on its trajectory, slowly approaching Sgr A*. Furthermore, we were able to detect S38, S42, S60, and S63 in a series of exposures for which we offset the fiber from Sgr A*. We provide an update on the orbits of all aforementioned stars. In addition to these known sources, the images also reveal a faint star moving to the west at a high angular velocity. We cannot find any coincidence with any known source and, thus, we refer to the new star as S300. From the flux ratio with S29, we estimate its K-band magnitude as $m_\mathrm{K}\left(\mathrm{S300}\right)\simeq 19.0 - 19.3$. Images obtained with CLEAN confirm the detection. △ Less

Submitted 14 December, 2021; originally announced December 2021.

Comments: 24 pages, 14 figures

Journal ref: A&A 657, A82 (2022)

The GRAVITY Young Stellar Object survey. VII. The inner dusty disks of T Tauri stars

Authors: The GRAVITY Collaboration, K. Perraut, L. Labadie, J. Bouvier, F. Ménard, L. Klarmann, C. Dougados, M. Benisty, J. -P. Berger, Y. -I. Bouarour, W. Brandner, A. Caratti o Garatti, P. Caselli, P. T. de Zeeuw, R. Garcia-Lopez, T. Henning, J. Sanchez-Bermudez, A. Sousa, E. van Dishoeck, E. Alécian, A. Amorim, Y. Clénet, R. Davies, A. Drescher, G. Duvert , et al. (33 additional authors not shown)

Abstract: These protoplanetary disks in T Tauri stars play a central role in star and planet formation. We spatially resolve at sub-au scales the innermost regions of a sample of T Tauri's disks to better understand their morphology and composition. We extended our homogeneous data set of 27 Herbig stars and collected near-IR K-band observations of 17 T Tauri stars, spanning effective temperatures and lumin… ▽ More These protoplanetary disks in T Tauri stars play a central role in star and planet formation. We spatially resolve at sub-au scales the innermost regions of a sample of T Tauri's disks to better understand their morphology and composition. We extended our homogeneous data set of 27 Herbig stars and collected near-IR K-band observations of 17 T Tauri stars, spanning effective temperatures and luminosities in the ranges of ~4000-6000 K and ~0.4-10 Lsun. We focus on the continuum emission and develop semi-physical geometrical models to fit the interferometric data and search for trends between the properties of the disk and the central star. The best-fit models of the disk's inner rim correspond to wide rings. We extend the Radius-luminosity relation toward the smallest luminosities (0.4-10 Lsun) and find the R~L^(1/2) trend is no longer valid, since the K-band sizes measured with GRAVITY are larger than the predicted sizes from sublimation radius computation. No clear correlation between the K-band half-flux radius and the mass accretion rate is seen. Having magnetic truncation radii in agreement with the K-band GRAVITY sizes would require magnetic fields as strong as a few kG, which should have been detected, suggesting that accretion is not the main process governing the location of the half-flux radius of the inner dusty disk. Our measurements agree with models that take into account the scattered light. The N-to-K band size ratio may be a proxy for disentangling disks with silicate features in emission from disks with weak and/or in absorption silicate features. When comparing inclinations and PA of the inner disks to those of the outer disks (ALMA) in nine objects of our sample, we detect misalignments for four objects. △ Less

Submitted 24 September, 2021; originally announced September 2021.

Comments: Accepted for publication in A&A; 20 pages, 15 figures, 9 tables

Journal ref: Astronomy & Astrophysics 655, A73 (2021)

The mass of Beta Pictoris c from Beta Pictoris b orbital motion

Authors: S. Lacour, J. J. Wang, L. Rodet, M. Nowak, J. Shangguan, H. Beust, A. -M. Lagrange, R. Abuter, A. Amorim, R. Asensio-Torres, M. Benisty, J. -P. Berger, S. Blunt, A. Boccaletti, A. Bohn, M. -L. Bolzer, M. Bonnefoy, H. Bonnet, G. Bourdarot, W. Brandner, F. Cantalloube, P. Caselli, B. Charnay, G. Chauvin, E. Choquet , et al. (74 additional authors not shown)

Abstract: We aim to demonstrate that the presence and mass of an exoplanet can now be effectively derived from the astrometry of another exoplanet. We combined previous astrometry of $β$ Pictoris b with a new set of observations from the GRAVITY interferometer. The orbital motion of $β$ Pictoris b is fit using Markov chain Monte Carlo simulations in Jacobi coordinates. The inner planet, $β$ Pictoris c, was… ▽ More We aim to demonstrate that the presence and mass of an exoplanet can now be effectively derived from the astrometry of another exoplanet. We combined previous astrometry of $β$ Pictoris b with a new set of observations from the GRAVITY interferometer. The orbital motion of $β$ Pictoris b is fit using Markov chain Monte Carlo simulations in Jacobi coordinates. The inner planet, $β$ Pictoris c, was also reobserved at a separation of 96\,mas, confirming the previous orbital estimations. From the astrometry of planet b only, we can (i) detect the presence of $β$ Pictoris c and (ii) constrain its mass to $10.04^{+4.53}_{-3.10}\,M_{\rm Jup}$. If one adds the astrometry of $β$ Pictoris c, the mass is narrowed down to $9.15^{+1.08}_{-1.06}\,M_{\rm Jup}$. The inclusion of radial velocity measurements does not affect the orbital parameters significantly, but it does slightly decrease the mass estimate to $8.89^{+0.75}_{-0.75}\,M_{\rm Jup}$. With a semimajor axis of $2.68\pm0.02$\,au, a period of $1221\pm15$ days, and an eccentricity of $0.32\pm0.02$, the orbital parameters of $β$ Pictoris c are now constrained as precisely as those of $β$ Pictoris b. The orbital configuration is compatible with a high-order mean-motion resonance (7:1). The impact of the resonance on the planets' dynamics would then be negligible with respect to the secular perturbations, which might have played an important role in the eccentricity excitation of the outer planet. △ Less

Submitted 22 September, 2021; originally announced September 2021.

Comments: Accepted in A&A Letter

Journal ref: A&A 654, L2 (2021)