Showing 1–50 of 154 results for author: Cunha, M S

Asteroseismic modelling of main-sequence solar-like stars and Kepler exoplanet host stars with the FICO procedure I. Catalogue of fundamental stellar properties

Authors: Jérôme Bétrisey, Daniel R. Reese, Camilla Pezzotti, Marie-Jo Goupil, Margarida S. Cunha

Abstract: We present detailed asteroseismic modelling of 95 main-sequence solar-like stars and Kepler exoplanet host stars using the FICO procedure, a three-step method that combines forward and inverse techniques that enables precise inference of fundamental stellar parameters such as mass, radius, age, and mean density. We applied the FICO procedure to a catalogue of stars with high-quality asteroseismic… ▽ More We present detailed asteroseismic modelling of 95 main-sequence solar-like stars and Kepler exoplanet host stars using the FICO procedure, a three-step method that combines forward and inverse techniques that enables precise inference of fundamental stellar parameters such as mass, radius, age, and mean density. We applied the FICO procedure to a catalogue of stars with high-quality asteroseismic and classical observations, and compared its results against literature values. We also compared its performance with direct frequency fitting using semi-empirical surface corrections. The FICO procedure achieved statistical precisions of 2.3%, 0.82%, 6.9%, and 0.49% in mass, radius, age, and mean density, respectively on average, well within PLATO quality requirements. We reconfirmed that surface-independent methods more effectively mitigate biases inherent to semi-empirical surface corrections, particularly for stars more massive than 1.15 Msun or above 6050 K. Two regimes were identified: near-solar conditions, where both approaches perform similarly, and higher-mass stars, where surface-independent methods consistently outperform direct fitting methods. While our results are consistent with literature values, we observed age biases (~11.5% on average for the Kepler LEGACY sample) that are comparable to the PLATO accuracy requirement of 10% for a Sun-like star, and therefore not negligible in that context. The FICO procedure provides a robust framework for high-precision stellar characterisation in the PLATO era. Its hybrid architecture effectively addresses surface effects, making it a promising tool for the accurate determination of exoplanet host-star properties. Our findings also highlight the importance of carefully selecting and validating the physical assumptions embedded in stellar models, particularly in the context of next-generation space missions such as PLATO. △ Less

Submitted 30 April, 2026; originally announced April 2026.

Comments: Accepted for publication in Astronomy and Astrophysics

Journal ref: A&A 710, A134 (2026)

Rapidly oscillating Ap stars observed with TESS. The LAMOST Ap sample and 49 Cam

Authors: Inês Rolo, Daniel L. Holdsworth, Margarida S. Cunha, Victoria Antoci, Donald W. Kurtz, Rahul Jayaraman, Ângela R. G. Santos

Abstract: The rapidly oscillating chemically peculiar A-type (roAp) stars offer valuable insights into the internal physical processes of all stars, but their study is challenged by their rarity. The large-scale TESS surveys have allowed for the collection of data for a sizeable dataset of roAp stars. Nevertheless, asteroseismic data obtained with TESS and Gaia has not been explored to its full potential. W… ▽ More The rapidly oscillating chemically peculiar A-type (roAp) stars offer valuable insights into the internal physical processes of all stars, but their study is challenged by their rarity. The large-scale TESS surveys have allowed for the collection of data for a sizeable dataset of roAp stars. Nevertheless, asteroseismic data obtained with TESS and Gaia has not been explored to its full potential. We develop an algorithm capable of analysing large quantities of data to search for new roAp stars and increase the current sample. We focus on data products that have not been previously explored for the search of roAp stars, namely the 200-s FFIs and 20-s cadence light curves. 20-s and 200-s cadence light curves of target stars are retrieved from the MAST server and cleaned. DFTs are computed for each light curve which are used to pre-whiten the data to remove any low frequency signals. A final DFT is calculated which is used to classify stars as non-pulsating (NP), delta Scuti or roAp based on the remaining signal. We apply our algorithm to two independent datasets: i) ~2700 Ap stars spectroscopically classified with LAMOST and observed by TESS in the 200-s FFIs and ii) all TESS 20-s cadence light curves available for known or candidate roAp stars. These two samples have no overlap, i.e. none of the LAMOST stars have been observed with 20-s cadence. We identify four new roAp stars: TIC 312111544, TIC 252881095, TIC 46054683, and 49 Cam (TIC 393276640). We find evidence in TESS data that TIC 252881095 may be part of a binary system. If the tentative ~30-d orbital signal is confirmed, TIC 252881095 could be one of the shortest-period roAp binary currently known. Furthermore, the detection of high-frequency pulsations in 49 Cam is particularly relevant, as this well-known roAp candidate star is here confirmed to be roAp based on TESS 20-s cadence data. △ Less

Submitted 21 April, 2026; originally announced April 2026.

Comments: Recommended for publication in the section "14. Catalogs and data" of Astronomy & Astrophysics

Journal ref: A&A 710, A50 (2026)

Traversable double-throat wormholes in a string cloud background

Authors: Yvens Amaral, M. S. Cunha, C. R. Muniz, M. O. Tahim

Abstract: This work constructs a new class of traversable wormhole solutions with a double-throat topology, modeled as a localized perturbation of the Ellis-Bronnikov metric in a string cloud background. Embedding diagrams and the analysis of curvature invariants, including the Kretschmann scalar and the Weyl invariant, illustrate the geometric transition from single to double-throat structures as a functio… ▽ More This work constructs a new class of traversable wormhole solutions with a double-throat topology, modeled as a localized perturbation of the Ellis-Bronnikov metric in a string cloud background. Embedding diagrams and the analysis of curvature invariants, including the Kretschmann scalar and the Weyl invariant, illustrate the geometric transition from single to double-throat structures as a function of the perturbation amplitude. By imposing the zero-tidal condition, we derive analytical expressions for the energy density and pressures, showing an asymptotic $r^{-2}$ decay characteristic of a string cloud, endowed with the topology of a global monopole. A key finding is that the energy density converges to a positive constant at the center, with the radial pressure becoming negative. This local behavior provides the repulsive support necessary to inflate the inter-throat region with non-exotic matter, concentrating Null Energy Condition violations to the throat vicinities. These results suggest that multi-throat geometries offer a natural mechanism for localizing exotic matter while maintaining a physical asymptotic background. △ Less

Submitted 5 May, 2026; v1 submitted 7 April, 2026; originally announced April 2026.

Comments: 18 pages, 14 figures, some references added, text improved

Assessment of PLATO Science Performance

Authors: Juan Cabrera, Heike Rauer, Reza Samadi, Valerio Nascimbeni, Anko Boerner, Denis Grießbach, Carsten Paproth, Martin Pertenaıs, Sami-M. Niemi, Szilard Csizmadia, Asier Abreu, Conny Aerts, Suzanne Aigrain, Matthias Ammler-von Eiff, Beatriz Aparicio del Moral, Thierry Appourchaux, David J. Armstrong, Ann Baeke, Gabor G. Balazs, Kevin Belkacem, Aaron Birch, Paz Bluhm, Tobias Boenke, Fabrice Boquet, Sam Bowling , et al. (99 additional authors not shown)

Abstract: The PLATO mission is scheduled for launch early 2027. In this paper we present an overview of the performance drivers for the mission at the time where all flight models of the cameras have been tested and integrated on the optical bench. The PLATO consortium needs an estimate of the planet detection yield to dimension the ground-based radial velocity follow-up resources. We provide updated estima… ▽ More The PLATO mission is scheduled for launch early 2027. In this paper we present an overview of the performance drivers for the mission at the time where all flight models of the cameras have been tested and integrated on the optical bench. The PLATO consortium needs an estimate of the planet detection yield to dimension the ground-based radial velocity follow-up resources. We provide updated estimates on the yield of planet detections that can be expected from the mission under certain assumptions. As of today, large uncertainties remain on the planet occurrence rates, especially for small planets in long-period orbits, and on our ability to detect these planets in the presence of stellar variability and instrumental noise. To partially overcome these limitations, we compare results using different planet occurrence rates, detectability rates, and we include an estimate on the expected contribution of stellar variability to the noise budget. The final detection yield of PLATO will provide constraints to planet occurrence rates which in turn will help constraining planet formation models. △ Less

Submitted 6 April, 2026; originally announced April 2026.

Comments: submitted to the special issue on PLATO in Experimental Astronomy

On regular black string spacetimes in nonlinear electrodynamics

Authors: G. Alencar, V. H. U. Borralho, T. M. Crispim, M. S. Cunha

Abstract: In this work, we investigate the coupling of General Relativity with Nonlinear Electrodynamics (NED), governed by a general Lagrangian $\mathcal{L}(\mathcal{F})$, to address the axial singularity of four-dimensional black strings. Through a model-independent analysis, we scrutinize the viability of regular configurations by extending no-go theorems, originally formulated for spherical spacetimes,… ▽ More In this work, we investigate the coupling of General Relativity with Nonlinear Electrodynamics (NED), governed by a general Lagrangian $\mathcal{L}(\mathcal{F})$, to address the axial singularity of four-dimensional black strings. Through a model-independent analysis, we scrutinize the viability of regular configurations by extending no-go theorems, originally formulated for spherical spacetimes, to cylindrical symmetries. We provide a comprehensive mathematical proof that regular, purely electric black strings cannot be generated by any NED Lagrangian that recovers the Maxwell limit in the weak-field regime, establishing a fundamental constraint for cylindrical topologies. Despite these limitations, we employ specific mathematical frameworks to construct new exact solutions for black strings, including cylindrical analogues of the well-known Bardeen and Hayward regular black hole classes. Each solution is analytically derived, and we demonstrate that their curvature invariants remain finite everywhere, effectively replacing the axial singularity with a regular core. Furthermore, we evaluate the physical consistency of these new metrics by subjecting them to stringent causality and unitarity constraints. Our results provide a comprehensive classification of the conditions under which NED can regularize cylindrical spacetimes and offer new insights into how topological differences between spherical and axial symmetries influence the global structure and the physical viability of non-singular gravitational objects in nonlinear gauge theories. △ Less

Submitted 17 July, 2026; v1 submitted 23 March, 2026; originally announced March 2026.

Cylindrically Symmetric Black Holes Sourced by Dekel-Zhao Dark Matter

Authors: G. Alencar, V. H. U. Borralho, M. S. Cunha, R. R. Landim

Abstract: In this work, we obtain analytical solutions for a $(3+1)$-dimensional black string and a $(2+1)$-dimensional black hole, both sourced by the Dekel-Zhao dark matter (DM) density profile. Our results indicate that the event horizon radius is sensitive to the inner slope parameter $a$; specifically, beyond a critical threshold, the horizon vanishes, leading to the formation of naked singularities. W… ▽ More In this work, we obtain analytical solutions for a $(3+1)$-dimensional black string and a $(2+1)$-dimensional black hole, both sourced by the Dekel-Zhao dark matter (DM) density profile. Our results indicate that the event horizon radius is sensitive to the inner slope parameter $a$; specifically, beyond a critical threshold, the horizon vanishes, leading to the formation of naked singularities. We find that the DM environment induces curvature singularities in the Ricci and Kretschmann scalars, which are absent in the vacuum BTZ case. Furthermore, an analysis of the effective energy-momentum tensor shows that while the null, weak, and strong energy conditions are strictly satisfied, the dominant energy condition is violated in the lower-dimensional scenario due to the high tangential pressure gradient. We also observe that DM modifies the Hawking temperature and free energy without compromising local or global stability. Notably, the DM distribution transforms the originally constant-curvature BTZ spacetime into a singular one, suggesting that a inherent stiffness of the DM profile is a determinant factor in the causal structure of these solutions. △ Less

Submitted 18 May, 2026; v1 submitted 10 February, 2026; originally announced February 2026.

Comments: 29 pages, 20 figures

Extra modes in helium-core-burning stars probing an infra core cavity

Authors: B. Mosser, M. Takata, C. Pinçon, M. S. Cunha, M. Vrard, K. Belkacem, S. Deheuvels, M. Matteuzzi

Abstract: Dipole mixed modes observed in the oscillation pattern of red giant stars probe the radiative regions in the stellar core. Oscillation spectra of helium-core-burning stars sometimes show extra peaks that remain unexplained by the dipole mixed-mode pattern expected from the coupling of a radiative cavity in the stellar core and a pressure cavity in the stellar envelope. We use the asymptotic expans… ▽ More Dipole mixed modes observed in the oscillation pattern of red giant stars probe the radiative regions in the stellar core. Oscillation spectra of helium-core-burning stars sometimes show extra peaks that remain unexplained by the dipole mixed-mode pattern expected from the coupling of a radiative cavity in the stellar core and a pressure cavity in the stellar envelope. We use the asymptotic expansion developed for a multi-cavity star in order to characterize these extra peaks. The analytical resonance condition of the multi-cavity gravito-acoustic modes, with two inner gravity cavities and an outer pressure cavity, helps us explain that the apparent extra peaks are dipole mixed modes that follow the 3-cavity oscillation pattern. The derivation of the two asymptotic period spacings associated with the two distinct regions in the radiative core provides an estimate of the full radiative cavity. Our results provide new constraints for analysing the overshoot or mixing in the core of helium-core-burning stars. An important structure discontinuity inside the radiative core may explain the larger than expected observed period spacings. △ Less

Submitted 20 January, 2026; originally announced January 2026.

Comments: Accepted in A&A

Coupled Oscillators, Frequency Transfer and the Higgs Mechanism's Teaching

Authors: M. O. Tahim, C. R. Muniz, M. S. Cunha, R. I. O. Junior

Abstract: In this work, we propose a simple model to teach the Higgs mechanism of spontaneous symmetry breaking in particle physics. The basic idea is associated with the analysis of two particles' motion subjected to specific potentials that, in certain situations, will reproduce oscillatory motions with a frequency transfer effect. In this work, we propose a simple model to teach the Higgs mechanism of spontaneous symmetry breaking in particle physics. The basic idea is associated with the analysis of two particles' motion subjected to specific potentials that, in certain situations, will reproduce oscillatory motions with a frequency transfer effect. △ Less

Submitted 21 August, 2025; v1 submitted 12 August, 2025; originally announced August 2025.

Comments: 17 pages, 6 figures, some typos corrected, some references fixed, text in Portuguese, accepted for publication in Rev. Bras. Ens. Fis

Traversable wormholes from a smoothed string fluid in 4D Einstein-Gauss-Bonnet gravity

Authors: C. R. Muniz, M. S. Cunha, L. C. N. Santos

Abstract: We investigate traversable wormhole solutions in four-dimensional Einstein-Gauss-Bonnet (EGB) gravity sourced by a smoothed string fluid. Originally proposed to model regular black holes, this energy density profile is adapted here to sustain wormhole geometries by allowing for a radially varying equation of state. We obtain zero-tidal-force solutions that satisfy all traversability criteria and r… ▽ More We investigate traversable wormhole solutions in four-dimensional Einstein-Gauss-Bonnet (EGB) gravity sourced by a smoothed string fluid. Originally proposed to model regular black holes, this energy density profile is adapted here to sustain wormhole geometries by allowing for a radially varying equation of state. We obtain zero-tidal-force solutions that satisfy all traversability criteria and remain globally regular. The Gauss-Bonnet (GB) coupling $α$ plays a central role in shaping the throat geometry. We identify a parameter region ($α\geq 1$, $\varepsilon \leq 0.1$) in which the null energy condition is satisfied in the vicinity of the throat, representing a significant improvement over general relativistic counterparts. The interplay between the smoothing scale $a$ and the string density $\varepsilon$ ensures finite curvature invariants while reducing the violation of energy conditions. An analysis of the volume integral quantifier and the complexity factor further shows that strong EGB coupling simultaneously suppresses gravitational complexity and the total amount of exotic matter. These results establish a unified framework in which the same string fluid source can generate both regular black holes and stable traversable wormholes, depending on the strength of higher-curvature corrections. △ Less

Submitted 16 January, 2026; v1 submitted 11 May, 2025; originally announced May 2025.

Comments: 23 pages, 21 figures, improved text, some new references added

Journal ref: Annals of Physics, 10 June 2026

Probing the Cores of Subdwarf B Stars: How do They Compare to Cores in Helium Core-Burning Red Giants?

Authors: Margarida S. Cunha, Juliana Amaral, Sofia Avelino, Anselmo Falorca, Yuri Damasceno, Pedro Avelino

Abstract: The mixing of material from stellar convective cores into their adjacent radiative layers has been a matter of long-standing debate. Pulsating subdwarf B stars offer excellent conditions to advance our understanding of this problem. In this work we use a model-independent approach to infer information about the cores of three subdwarf B stars and compare it with similar inferences from earlier ana… ▽ More The mixing of material from stellar convective cores into their adjacent radiative layers has been a matter of long-standing debate. Pulsating subdwarf B stars offer excellent conditions to advance our understanding of this problem. In this work we use a model-independent approach to infer information about the cores of three subdwarf B stars and compare it with similar inferences from earlier analysis of red giants in the helium core-burning phase. This is achieved by fitting an analytical description of the gravity-mode pulsation periods to pulsation data collected by the Kepler satellite. From the fits we infer the reduced asymptotic period spacings and the amplitude and position of sharp structural variations associated with chemical discontinuities in the stellar interiors. Our results indicate the presence of sharp structural variations with similar properties in all three stars, located near the edge of the gravity-mode propagation cavity and likely associated with the C-O/He transition. We find that these structural variations differ systematically from those of helium core-burning red giant stars, having larger amplitudes and being located at a larger buoyancy radius. This suggests that chemical mixing beyond the adiabatically stratified core into the radiatively stratified layers may be more extensive in subdwarf B stars than in helium core-burning red giants. Alternatively, the stratification of the mixing region beyond the adiabatically stratified core may differ significantly between the two types of stars. The model-independent constraints set on the structural variations inside these three stars are the first of a kind and will be key to enhance the modelling of layers adjacent to stellar convective cores and to test non-canonical stellar evolution channels leading to the formation of hot subdwarf stars. △ Less

Submitted 2 May, 2025; originally announced May 2025.

Comments: Accepted for publication in Astronomy and Astrophysics

Journal ref: A&A 699, A111 (2025)

Five dimensional rotating and Quintessence black hole and their hypershadows

Authors: Milko Estrada, L. C. N. Santos, M. S. Cunha

Abstract: We present a new five-dimensional rotating quintessence black hole solution. To obtain this, we employ the $5D$ version of the Janis Newman algorithm, which incorporates the Hopf bifurcation. The variation of the quintessence parameter $w_q$ causes the geometry to transition from a regular rotating universe surrounded by a cosmological horizon to a singular rotating geometry, which can represent a… ▽ More We present a new five-dimensional rotating quintessence black hole solution. To obtain this, we employ the $5D$ version of the Janis Newman algorithm, which incorporates the Hopf bifurcation. The variation of the quintessence parameter $w_q$ causes the geometry to transition from a regular rotating universe surrounded by a cosmological horizon to a singular rotating geometry, which can represent a naked singularity, a singular extremal black hole, or a singular black hole with both an inner and an outer (event) horizon. We have also determined the properties of the ergosphere. For the study of the shadow, we followed a novel approach in which the $2D$ shadow observed by humans corresponds to cross sections of the $3D$ shadow. We analyzed how quintessence affects both the size and shape of the black hole shadow, showing that increasing the quintessence strength reduces the shadow radius, contrary to the known results in $4D$. We also propose a speculative methodology to test the shadow behavior in five dimensional scenarios, in light of the constraints provided by the Event Horizon Telescope (EHT) concerning the shadow of the four-dimensional supermassive black hole M87. We identify scenarios in which the theoretical $5D$ results could be consistent with these observational constraints. We have also tested the circularity deviation of our shadows, finding that the results satisfy the bound $ΔC \leq 0.1$ both in the case with quintessence and in the limiting case without quintessence. Finally, we determine the energy conditions required to support the solution. △ Less

Submitted 24 July, 2025; v1 submitted 15 April, 2025; originally announced April 2025.

Comments: Accepted in Physics of the Dark Universe

Journal ref: Phys.Dark Univ. 49 (2025) 102041

Signature of spin-down stalling in stellar magnetic activity. The case of the open cluster NGC 6811

Authors: Ângela R. G. Santos, Diego Godoy-Rivera, Savita Mathur, Sylvain N. Breton, Rafael A. García, Margarida S. Cunha

Abstract: Stellar rotation and magnetic activity have a complex evolution that reveals multiple regimes. One of the related transitions that is seen in the rotation distribution for main-sequence (MS) solar-like stars has been attributed to core-envelope coupling and the consequent angular-momentum transfer between a fast core and a slow envelope. This feature is known as spin-down stalling and is related t… ▽ More Stellar rotation and magnetic activity have a complex evolution that reveals multiple regimes. One of the related transitions that is seen in the rotation distribution for main-sequence (MS) solar-like stars has been attributed to core-envelope coupling and the consequent angular-momentum transfer between a fast core and a slow envelope. This feature is known as spin-down stalling and is related to the intermediate-rotation gap seen in field stars. Beyond this rotation signature, we search for evidence of it in stellar magnetic activity. We investigated the magnetic activity of the 1 Gyr old NGC 6811, a Kepler-field cluster, and Kepler MS stars of different ages. The magnetic activity was measured through the photometric magnetic activity proxy, Sph. To characterize the evolution of the magnetic activity for the Kepler sample, we split it according to the relative rotation and computed the respective activity sequences. We found the signature of core-envelope coupling in the magnetic activity of NGC 6811 and in the Kepler MS sample. In NGC 6811, we found enhanced magnetic activity for a range of effective temperatures that remained for significant timescales. In the Kepler sample, the magnetic activity sequences pile up in two distinct regions: at high activity levels that coincide with stars near the stalling mentioned above, where a behavior inversion is observed (slowly rotating stars have higher activity levels than fast-rotating stars, which is opposite to the overall behavior); and at low activity levels corresponding to slow rotators close to the detection limit, potentially facing a weakening of the magnetic braking. These results support the recent proposition that the strong shear experienced by stars during the core-envelope coupling phase can cause enhanced activity. This study helps us to shed light on the interplay between rotation, magnetic activity, and their evolution. △ Less

Submitted 7 April, 2025; originally announced April 2025.

Comments: 7 pages, 5 figures (+ Appendixes: 7 pages, 8 figures, 1 table). Accepted for publication in A&A

Journal ref: A&A 697, A177 (2025)

Black holes inside cosmic voids

Authors: Francisco Bento Lustosa, Milko Estrada, Marcony S. Cunha, Celio R. Muniz

Abstract: This study examines the gravitational and thermodynamic properties of static, spherically symmetric black holes within cosmic voids -- vast underdense regions of the universe. By deriving a novel solution based on a universal density profile for voids, we analyze its spacetime structure, which reveals two horizons: One of the black hole and the other related to the de Sitter-like behavior. As the… ▽ More This study examines the gravitational and thermodynamic properties of static, spherically symmetric black holes within cosmic voids -- vast underdense regions of the universe. By deriving a novel solution based on a universal density profile for voids, we analyze its spacetime structure, which reveals two horizons: One of the black hole and the other related to the de Sitter-like behavior. As the void approaches a perfect vacuum, the black hole horizon diminishes, tending to that of the Schwarzschild solution, while the outer horizon increases. We also study the solution stability via sound speed of the fluid, as well as the thermodynamic properties, including Hawking temperature, evaporation time, entropy, and specific heat. Our results show that as the void empties, the Hawking temperature rises, shortening evaporation times. The entropy follows the area's law and specific heat exhibits a minimum for a given black hole size, indicating a thermal transition and highlighting the role of voids in the black hole evolution. These findings offer new insights into the relationship between local gravitational collapse and large-scale cosmic structure, enhancing our understanding of the black hole behavior in underdense environments. We also provide a glimpse of a potential thermodynamic interaction between the event horizon and the cosmological horizon. △ Less

Submitted 7 December, 2025; v1 submitted 17 March, 2025; originally announced March 2025.

Comments: 22 pages, 9 figures

Journal ref: JCAP 05 (2025) 052

On the seismic modelling of subgiant stars: testing different grid interpolation methods

Authors: Miguel Clara, Margarida S. Cunha, Pedro P. Avelino, Tiago L. Campante, Sébastien Deheuvels, Daniel R. Reese

Abstract: Context. The emergence of mixed modes during the subgiant phase, whose frequencies are characterized by a fast evolution with age, can potentially enable a precise determination of stellar properties, a key goal for future missions like PLATO. However, current modelling techniques often consider grids that lack the resolution to properly account for the fast mode frequency evolution, consequently… ▽ More Context. The emergence of mixed modes during the subgiant phase, whose frequencies are characterized by a fast evolution with age, can potentially enable a precise determination of stellar properties, a key goal for future missions like PLATO. However, current modelling techniques often consider grids that lack the resolution to properly account for the fast mode frequency evolution, consequently requiring the use of interpolation algorithms to cover the parameter space in between the grid models when applying model-data comparison methods. Aims. We aim at reproducing the $\ell$=1 mode frequencies within the accuracy limits associated with the typical observational errors ($\sim$0.1 $μ$Hz) through interpolation on a grid of subgiant models. Methods. With that aim, we used variations of a two-step interpolation algorithm, which considered linear and cubic splines interpolation methods and different age proxies (physical age, scaled age, and central density). Results. The best results were obtained using an algorithm that considers cubic splines interpolation along tracks, linear interpolation across tracks, and central density $ρ_\text{c}$ as the age proxy. This combination yielded, on average, an absolute error of 0.14 $μ$Hz, but reached maximum absolute errors on the interpolated frequencies of 1.2 $μ$Hz for some models, which is an order of magnitude higher than the typical observational errors. Furthermore, we investigated the impact on the accuracy of the interpolation from changes in the physical properties of the stars, showing, in particular, how the addition of core overshoot can affect significantly the interpolation results. △ Less

Submitted 16 January, 2025; originally announced January 2025.

Comments: Accepted for publication in Astronomy & Astrophysics (18 pages, 16 figures)

Glitches in solar-like oscillating F-type stars: Possible contribution of non-linear terms

Authors: M. Deal, M. -J. Goupil, J. Philidet, M. S. Cunha, R. Teissonniere, E. Josselin

Abstract: The glitch signatures in $r_{010}$ for F-type stars (higher amplitude and period of the oscillatory component) are very different from those of G-type stars. The aim of this work is to analyse the signatures of these glitches and understand the origin of the differences in these signatures between G-type and F-type stars. We fit the glitch signatures in the frequencies, second differences, and… ▽ More The glitch signatures in $r_{010}$ for F-type stars (higher amplitude and period of the oscillatory component) are very different from those of G-type stars. The aim of this work is to analyse the signatures of these glitches and understand the origin of the differences in these signatures between G-type and F-type stars. We fit the glitch signatures in the frequencies, second differences, and $r_{010}$ ratios while assuming either a sinusoidal variation or a more complex expression. The fit provides the acoustic depth, and hence the position, of the bottom of the convective envelope for nine \textit{Kepler} stars and the Sun. We find that for F-type stars, the most commonly used fitting expressions for the glitch of the bottom of the convective envelope provide different measurements of the position of the bottom of the convective envelope for the three seismic indicators, while it is not the case for G-type stars. When adding an additional term in the fitting expression with twice the acoustic depth of the standard term (a contribution that accounts for the highly non-sinusoidal shape of the signature in the $r_{010}$ ratios), we find better agreement between the three seismic indicators and with the prediction of stellar evolution models. While the origin of this additional term is not yet understood, this may be an indication that the transition between the convective envelope and the underlying radiative zone is different for G- and F-type stars. This outcome brings new insight into the physics in these regions. △ Less

Submitted 19 December, 2024; originally announced December 2024.

Comments: 20 pages, 19 figures, 2 tables, accepted for publication in A&A

HD 60435: The star that stopped pulsating

Authors: Donald W. Kurtz, Gerald Handler, Daniel L. Holdsworth, Margarida S. Cunha, Hideyuki Saio, Thebe Medupe, Simon J. Murphy, Joachim Krüger, E. Brunsden, Victoria Antoci, Daniel R. Hey, Noi Shitrit, Jaymie M. Matthews

Abstract: HD 60435 is a well-known rapidly oscillating (roAp) Ap star with a series of alternating even and odd degree modes, making it a prime asteroseismic target. It is also an oblique pulsator with rotational inclination, $i$, and magnetic/pulsation obliquity, $β$, such that both magnetic/pulsation poles are viewed over the rotation period, $P_{\rm rot} = 7.679696$ d, determined from rotational light va… ▽ More HD 60435 is a well-known rapidly oscillating (roAp) Ap star with a series of alternating even and odd degree modes, making it a prime asteroseismic target. It is also an oblique pulsator with rotational inclination, $i$, and magnetic/pulsation obliquity, $β$, such that both magnetic/pulsation poles are viewed over the rotation period, $P_{\rm rot} = 7.679696$ d, determined from rotational light variations. While some roAp stars have stable pulsation mode amplitudes over decades, HD 60435 is known to have amplitude variations on time scales as short as 1 d. We show from 5 yr of {\it TESS} observations that there is strong amplitude modulation on this short time scale with possible mode interactions. Most remarkably, HD 60435 stopped pulsating during the time span of the {\it TESS} observations. This is the first time that any pulsating star has been observed to cease pulsating entirely. That has implications for mode interaction, excitation and damping, and is relevant to the problem of why only some stars in many pulsation instability strips pulsate, while others do not. During a 24.45-d time span of the {\it TESS} data when there was mode stability for a dipole mode and a quadrupole mode, the oblique pulsator model constrained $i$ and $β$, which we used to model those modes with a magnetic pulsation model from which we determined a polar field strength of 4 kG, in good agreement with a known magnetic measurement. We modelled the frequency separations showing that they can constrain the global metallicity, something that is not possible from spectroscopy of the highly peculiar Ap atmosphere. △ Less

Submitted 6 December, 2024; originally announced December 2024.

Comments: 19 pages plus an appendix. 13 figures. Accepted for publication in MNRAS

Static and stationary loop quantum black bounces

Authors: C. R. Muniz, G. Alencar, M. S. Cunha, Gonzalo J. Olmo

Abstract: We explore a static and stationary black bounce geometry inspired by Loop Quantum Gravity (LQG), focusing on how LQG corrections and regularization parameters affect its properties. Building on the spherically symmetric and static black hole solution from \cite{Kelly:2020uwj}, we trace its origin to Non-Linear Electrodynamics (NED) with electric and magnetic charges and check the energy conditions… ▽ More We explore a static and stationary black bounce geometry inspired by Loop Quantum Gravity (LQG), focusing on how LQG corrections and regularization parameters affect its properties. Building on the spherically symmetric and static black hole solution from \cite{Kelly:2020uwj}, we trace its origin to Non-Linear Electrodynamics (NED) with electric and magnetic charges and check the energy conditions (NEC, WEC, SEC). By extending the geometry using the Simpson-Visser procedure, we construct a black hole-wormhole bounce structure, influenced by LQG parameters. We analyze the horizon structure to constrain parameters for black holes and wormholes, and examine curvature and new sources including a phantom-type scalar field to ensure spacetime regularity and adherence to energy conditions. Thermodynamic properties are also studied, revealing the existence of remnants and phase transitions. Additionally, we derive a rotating black bounce solution, verifying its regularity, and putting forward that the spherical bouncing surface turns into an ellipsoid with no ring singularity. Finally, we find that increasing the LQG parameter leads to smaller ergospheres and reduced shadows, with potential implications for observational astrophysics and quantum gravitational signatures. △ Less

Submitted 16 August, 2024; originally announced August 2024.

Comments: 24 pages, 20 figures

Buoyancy glitches in pulsating stars revisited

Authors: Margarida S. Cunha, Yuri C. Damasceno, Juliana Amaral, Anselmo Falorca, Jørgen Christensen-Dalsgaard, Pedro P. Avelino

Abstract: Sharp structural variations induce specific signatures on stellar pulsations that can be studied to infer localised information on the stratification of the star. This information is key to improve our understanding of the physical processes that lead to the structural variations and how to model them. Here we revisit and extend the analysis of the signature of different types of buoyancy glitches… ▽ More Sharp structural variations induce specific signatures on stellar pulsations that can be studied to infer localised information on the stratification of the star. This information is key to improve our understanding of the physical processes that lead to the structural variations and how to model them. Here we revisit and extend the analysis of the signature of different types of buoyancy glitches in gravity-mode and mixed-mode pulsators presented in earlier works, including glitches with step-like, Gaussian-like, and Dirac-$δ$-like shapes. In particular, we provide analytical expressions for the perturbations to the periods and show that these can be reliably used in place of the expressions provided for the period spacings, with the advantage that the use of the new expressions does not require modes with consecutive radial orders to be observed. Based on a comparison with two limit cases and on simulated data, we further tested the accuracy of the expression for the Gaussian-like glitch signature whose derivation in an earlier work involved a significant approximation. We find that the least reliable glitch parameter inferred from fitting that expression is the amplitude, which can be up to a factor of two larger than the true amplitude, reaching this limit when the glitch is small. We further discuss the impact on the glitch signature of considering a glitch in the inner and outer half of the g-mode cavity, emphasising the break of symmetry that takes place in the case of mixed-mode pulsators. △ Less

Submitted 19 April, 2024; originally announced April 2024.

Comments: 16 pages, accepted for publication in Astronomy & Astrophysics

Journal ref: A&A 687, A100 (2024)

Expanding the frontiers of cool-dwarf asteroseismology with ESPRESSO. Detection of solar-like oscillations in the K5 dwarf $ε$ Indi

Authors: T. L. Campante, H. Kjeldsen, Y. Li, M. N. Lund, A. M. Silva, E. Corsaro, J. Gomes da Silva, J. H. C. Martins, V. Adibekyan, T. Azevedo Silva, T. R. Bedding, D. Bossini, D. L. Buzasi, W. J. Chaplin, R. R. Costa, M. S. Cunha, E. Cristo, J. P. Faria, R. A. García, D. Huber, M. S. Lundkvist, T. S. Metcalfe, M. J. P. F. G. Monteiro, A. W. Neitzel, M. B. Nielsen , et al. (3 additional authors not shown)

Abstract: Fuelled by space photometry, asteroseismology is vastly benefitting the study of cool main-sequence stars, which exhibit convection-driven solar-like oscillations. Even so, the tiny oscillation amplitudes in K dwarfs continue to pose a challenge to space-based asteroseismology. A viable alternative is offered by the lower stellar noise over the oscillation timescales in Doppler observations. In th… ▽ More Fuelled by space photometry, asteroseismology is vastly benefitting the study of cool main-sequence stars, which exhibit convection-driven solar-like oscillations. Even so, the tiny oscillation amplitudes in K dwarfs continue to pose a challenge to space-based asteroseismology. A viable alternative is offered by the lower stellar noise over the oscillation timescales in Doppler observations. In this letter we present the definite detection of solar-like oscillations in the bright K5 dwarf $ε$ Indi based on time-intensive observations collected with the ESPRESSO spectrograph at the VLT, thus making it the coolest seismic dwarf ever observed. We measured the frequencies of a total of 19 modes of degree $\ell=0$--2 along with $ν_{\rm max}=5305\pm176\:{\rm μHz}$ and $Δν=201.25\pm0.16\:{\rm μHz}$. The peak amplitude of radial modes is $2.6\pm0.5\:{\rm cm\,s^{-1}}$, or a mere ${\sim} 14\%$ of the solar value. Measured mode amplitudes are ${\sim} 2$ times lower than predicted from a nominal $L/M$ scaling relation and favour a scaling closer to $(L/M)^{1.5}$ below ${\sim} 5500\:{\rm K}$, carrying important implications for our understanding of the coupling efficiency between pulsations and near-surface convection in K dwarfs. This detection conclusively shows that precise asteroseismology of cool dwarfs is possible down to at least the mid-K regime using next-generation spectrographs on large-aperture telescopes, effectively opening up a new domain in observational asteroseismology. △ Less

Submitted 24 March, 2024; originally announced March 2024.

Comments: Accepted for publication in Astronomy & Astrophysics Letters

Journal ref: A&A, 683, L16 (2024)

TESS Cycle 2 observations of roAp stars with 2-min cadence data

Authors: D. L. Holdsworth, M. S. Cunha, M. Lares-Martiz, D. W. Kurtz, V. Antoci, S. Barceló Forteza, P. De Cat, A. Derekas, C. Kayhan, D. Ozuyar, M. Skarka, D. R. Hey, F. Shi, D. M. Bowman, O. Kobzar, A. Ayala Gómez, Zs. Bognár, D. L. Buzasi, M. Ebadi, L. Fox-Machado, A. García Hernández, H. Ghasemi, J. A. Guzik, R. Handberg, G. Handler , et al. (24 additional authors not shown)

Abstract: We present the results of a systematic search of the Transiting Exoplanet Survey Satellite (TESS) 2-min cadence data for new rapidly oscillating Ap (roAp) stars observed during the Cycle 2 phase of its mission. We find seven new roAp stars previously unreported as such and present the analysis of a further 25 roAp stars that are already known. Three of the new stars show multiperiodic pulsations,… ▽ More We present the results of a systematic search of the Transiting Exoplanet Survey Satellite (TESS) 2-min cadence data for new rapidly oscillating Ap (roAp) stars observed during the Cycle 2 phase of its mission. We find seven new roAp stars previously unreported as such and present the analysis of a further 25 roAp stars that are already known. Three of the new stars show multiperiodic pulsations, while all new members are rotationally variable stars, leading to almost 70 per cent (22) of the roAp stars presented being $α^2$ CVn-type variable stars. We show that targeted observations of known chemically peculiar stars are likely to overlook many new roAp stars, and demonstrate that multi-epoch observations are necessary to see pulsational behaviour changes. We find a lack of roAp stars close to the blue edge of the theoretical roAp instability strip, and reaffirm that mode instability is observed more frequently with precise, space-based observations. In addition to the Cycle 2 observations, we analyse TESS data for all known roAp stars. This amounts to 18 further roAp stars observed by TESS. Finally, we list six known roAp stars that TESS is yet to observe. We deduce that the incidence of roAp stars amongst the Ap star population is just 5.5 per cent, raising fundamental questions about the conditions required to excite pulsations in Ap stars. This work, coupled with our previous work on roAp stars in Cycle 1 observations, presents the most comprehensive, homogeneous study of the roAp stars in the TESS nominal mission, with a collection of 112 confirmed roAp stars in total. △ Less

Submitted 7 December, 2023; originally announced December 2023.

Comments: Accepted for publication in MNRAS. 32 Pages, 2 Tables, 77 Figures

TESS Giants Transiting Giants V -- Two hot Jupiters orbiting red-giant hosts

Authors: Filipe Pereira, Samuel K. Grunblatt, Angelica Psaridi, Tiago L. Campante, Margarida S. Cunha, Nuno C. Santos, Diego Bossini, Daniel Thorngren, Coel Hellier, François Bouchy, Monika Lendl, Dany Mounzer, Stéphane Udry, Corey Beard, Casey L. Brinkman, Howard Isaacson, Samuel N. Quinn, Dakotah Tyler, George Zhou, Steve B. Howell, Andrew W. Howard, Jon M. Jenkins, Sara Seager, Roland K. Vanderspek, Joshua N. Winn , et al. (2 additional authors not shown)

Abstract: In this work we present the discovery and confirmation of two hot Jupiters orbiting red-giant stars, TOI-4377 b and TOI-4551 b, observed by TESS in the southern ecliptic hemisphere and later followed-up with radial-velocity (RV) observations. For TOI-4377 b we report a mass of $0.957^{+0.089}_{-0.087} \ M_\mathrm{J}$ and a inflated radius of $1.348 \pm 0.081 \ R_\mathrm{J}$ orbiting an evolved int… ▽ More In this work we present the discovery and confirmation of two hot Jupiters orbiting red-giant stars, TOI-4377 b and TOI-4551 b, observed by TESS in the southern ecliptic hemisphere and later followed-up with radial-velocity (RV) observations. For TOI-4377 b we report a mass of $0.957^{+0.089}_{-0.087} \ M_\mathrm{J}$ and a inflated radius of $1.348 \pm 0.081 \ R_\mathrm{J}$ orbiting an evolved intermediate-mass star ($1.36 \ \mathrm{M}_\odot$, $3.52 \ \mathrm{R}_\odot$; TIC 394918211) on a period of of $4.378$ days. For TOI-4551 b we report a mass of $1.49 \pm 0.13 \ M_\mathrm{J}$ and a radius that is not obviously inflated of $1.058^{+0.110}_{-0.062} \ R_\mathrm{J}$, also orbiting an evolved intermediate-mass star ($1.31 \ \mathrm{M}_\odot$, $3.55 \ \mathrm{R}_\odot$; TIC 204650483) on a period of $9.956$ days. We place both planets in context of known systems with hot Jupiters orbiting evolved hosts, and note that both planets follow the observed trend of the known stellar incident flux-planetary radius relation observed for these short-period giants. Additionally, we produce planetary interior models to estimate the heating efficiency with which stellar incident flux is deposited in the planet's interior, estimating values of $1.91 \pm 0.48\%$ and $2.19 \pm 0.45\%$ for TOI-4377 b and TOI-4551 b respectively. These values are in line with the known population of hot Jupiters, including hot Jupiters orbiting main sequence hosts, which suggests that the radii of our planets have reinflated in step with their parent star's brightening as they evolved into the post-main-sequence. Finally, we evaluate the potential to observe orbital decay in both systems. △ Less

Submitted 11 November, 2023; originally announced November 2023.

Comments: 14 pages with 8 figures and 6 tables. Accepted for publication in the Monthly Notices of the Royal Astronomical Society

Systematics in Asteroseismic Modelling: Application of a Correlated Noise Model for Oscillation Frequencies

Authors: Tanda Li, Guy R. Davies, Martin Nielsen, Margarida S. Cunha, Alexander J. Lyttle

Abstract: The detailed modelling of stellar oscillations is a powerful approach to characterising stars. However, poor treatment of systematics in theoretical models leads to misinterpretations of stars. Here we propose a more principled statistical treatment for the systematics to be applied to fitting individual mode frequencies with a typical stellar model grid. We introduce a correlated noise model base… ▽ More The detailed modelling of stellar oscillations is a powerful approach to characterising stars. However, poor treatment of systematics in theoretical models leads to misinterpretations of stars. Here we propose a more principled statistical treatment for the systematics to be applied to fitting individual mode frequencies with a typical stellar model grid. We introduce a correlated noise model based on a Gaussian Process (GP) kernel to describe the systematics given that mode frequency systematics are expected to be highly correlated. We show that tuning the GP kernel can reproduce general features of frequency variations for changing model input physics and fundamental parameters. Fits with the correlated noise model better recover stellar parameters than traditional methods which either ignore the systematics or treat them as uncorrelated noise. △ Less

Submitted 4 June, 2023; originally announced June 2023.

Comments: 11 pages

Journal ref: 2023MNRAS.523...80L

Temporal variation of the photometric magnetic activity for the Sun and Kepler solar-like stars

Authors: A. R. G. Santos, S. Mathur, R. A. García, A. -M. Broomhall, R. Egeland, A. Jiménez, D. Godoy-Rivera, S. N. Breton, Z. R. Claytor, T. S. Metcalfe, M. S. Cunha, L. Amard

Abstract: The photometric time series of solar-like stars can exhibit rotational modulation due to active regions co-rotating with the stellar surface, allowing us to constrain stellar rotation and magnetic activity. In this work we investigate the behavior, particularly the variability, of the photometric magnetic activity of Kepler solar-like stars and compare it with that of the Sun. We adopted the photo… ▽ More The photometric time series of solar-like stars can exhibit rotational modulation due to active regions co-rotating with the stellar surface, allowing us to constrain stellar rotation and magnetic activity. In this work we investigate the behavior, particularly the variability, of the photometric magnetic activity of Kepler solar-like stars and compare it with that of the Sun. We adopted the photometric magnetic activity proxy Sph, which was computed with a cadence of 5 x the rotation period, Prot. The average Sph was taken as the mean activity level, and the standard deviation was taken as a measure of the temporal variation of the magnetic activity over the observations. We also analyzed Sun-as-a-star photometric data from VIRGO. Sun-like stars were selected from a very narrow parameter space around the solar properties. We also looked into KIC 8006161 (HD 173701), an active metal-rich G dwarf, and we compared its magnetic activity to that of stars with similar stellar parameters. We find that the amplitude of Sph variability is strongly correlated with its mean value, independent of spectral type. An equivalent relationship has been found for ground-based observations of chromospheric activity emission and magnetic field strength, but in this work we show that photometric Kepler data also present the same behavior. While, depending on the cycle phase, the Sun is among the less active stars, we find that the solar Sph properties are consistent with those observed in Kepler Sun-like stars. KIC 8006161 is, however, among the most active of its peers, which tend to be metal-rich. This results from an underlying relationship between Prot and metallicity and supports the following interpretation of the magnetic activity of KIC 8006161: its strong activity is a consequence of its high metallicity, which affects the depth of the convection zone and, consequently, the efficiency of the dynamo. △ Less

Submitted 6 April, 2023; originally announced April 2023.

Comments: Published in A&A; 12 pages including 11 figures and 3 tables (main text); 10 additional pages including 17 figures and 5 tables (appendix)

Journal ref: A&A (2023), 672, A56

Revisiting the Red-giant Branch Hosts KOI-3886 and $ι$ Draconis. Detailed Asteroseismic Modeling and Consolidated Stellar Parameters

Authors: Tiago L. Campante, Tanda Li, J. M. Joel Ong, Enrico Corsaro, Margarida S. Cunha, Timothy R. Bedding, Diego Bossini, Sylvain N. Breton, Derek L. Buzasi, William J. Chaplin, Morgan Deal, Rafael A. García, Michelle L. Hill, Marc Hon, Daniel Huber, Chen Jiang, Stephen R. Kane, Cenk Kayhan, James S. Kuszlewicz, Jorge Lillo-Box, Savita Mathur, Mário J. P. F. G. Monteiro, Filipe Pereira, Nuno C. Santos, Aldo Serenelli , et al. (1 additional authors not shown)

Abstract: Asteroseismology is playing an increasingly important role in the characterization of red-giant host stars and their planetary systems. Here, we conduct detailed asteroseismic modeling of the evolved red-giant branch (RGB) hosts KOI-3886 and $ι$ Draconis, making use of end-of-mission Kepler (KOI-3886) and multi-sector TESS ($ι$ Draconis) time-series photometry. We also model the benchmark star KIC… ▽ More Asteroseismology is playing an increasingly important role in the characterization of red-giant host stars and their planetary systems. Here, we conduct detailed asteroseismic modeling of the evolved red-giant branch (RGB) hosts KOI-3886 and $ι$ Draconis, making use of end-of-mission Kepler (KOI-3886) and multi-sector TESS ($ι$ Draconis) time-series photometry. We also model the benchmark star KIC 8410637, a member of an eclipsing binary, thus providing a direct test to the seismic determination. We test the impact of adopting different sets of observed modes as seismic constraints. Inclusion of $\ell=1$ and 2 modes improves the precision on the stellar parameters, albeit marginally, compared to adopting radial modes alone, with $1.9$-$3.0\%$ (radius), $5$-$9\%$ (mass), and $19$-$25\%$ (age) reached when using all p-dominated modes as constraints. Given the very small spacing of adjacent dipole mixed modes in evolved RGB stars, the sparse set of observed g-dominated modes is not able to provide extra constraints, further leading to highly multimodal posteriors. Access to multi-year time-series photometry does not improve matters, with detailed modeling of evolved RGB stars based on (lower-resolution) TESS data sets attaining a precision commensurate with that based on end-of-mission Kepler data. Furthermore, we test the impact of varying the atmospheric boundary condition in our stellar models. We find mass and radius estimates to be insensitive to the description of the near-surface layers, at the expense of substantially changing both the near-surface structure of the best-fitting models and the values of associated parameters like the initial helium abundance, $Y_{\rm i}$. Attempts to measure $Y_{\rm i}$ from seismic modeling of red giants may thus be systematically dependent on the choice of atmospheric physics. △ Less

Submitted 4 April, 2023; originally announced April 2023.

Comments: Accepted for publication in The Astronomical Journal (AJ)

Glitches in solar-like oscillating F-type stars: Theoretical signature of the base of the convective envelope on the ratios $r_{010}$

Authors: M. Deal, M. -J. Goupil, M. S. Cunha, M. J. P. F. G. Monteiro, Y. Lebreton, S. Christophe, F. Pereira, R. Samadi, A. V. Oreshina, G. Buldgen

Abstract: The transition between convective and radiative stellar regions is still not fully understood. The sharp variations in sound speed located in these transition regions give rise to a signature in specific seismic indicators, opening the possibility to constrain the physics of convection to radiation transition. Among those seismic indicators, the ratios of the small to large frequency separation fo… ▽ More The transition between convective and radiative stellar regions is still not fully understood. The sharp variations in sound speed located in these transition regions give rise to a signature in specific seismic indicators, opening the possibility to constrain the physics of convection to radiation transition. Among those seismic indicators, the ratios of the small to large frequency separation for $l=0$ and $1$ modes ($r_{010}$) were shown to be particularly efficient to probe these transition regions. Interestingly, in the Kepler Legacy F-type stars, the oscillatory signatures left in the $r_{010}$ ratios by the sharp sound-speed variation have unexpected large amplitudes that still need to be explained. We show that the signature of the bottom of the convective envelope is amplified in the ratios $r_{010}$ by the frequency dependence of the amplitude compared to the signal seen in the frequencies themselves or the second differences. We find that among the different options of physical input investigated here, large amplitude signatures can only be obtained when convective penetration of the surface convective zone into the underlying radiative region is taken into account. In this case and even for amplitudes as large as those observed in F-type stars, the oscillating signature in the ratios can only be detected when the convective envelope is deep enough. This deep extension of the convective envelope causes doubt that the origin of the large amplitudes is due to penetrative convection as it is modelled here or implies that current stellar modelling (without penetrative convection) leads to an underestimation of the size of convective envelopes. In any case, studying the glitch signatures of a large number of oscillating F-type stars opens the possibility to constrain the physics of the stellar interior in these regions. △ Less

Submitted 24 March, 2023; originally announced March 2023.

Comments: 19 pages, 12 figures, accepted for publication in A&A

Journal ref: A&A 673, A49 (2023)

FRA -- A new Fast, Robust and Automated pipeline for the detection and measurement of solar-like oscillations in time-series photometry of red-giant stars

Authors: C. Gehan, T. L. Campante, M. S. Cunha, F. Pereira

Abstract: We developed, tested and validated a new Fast, Robust and Automated (FRA) tool to detect solar-like oscillations. FRA is based on the detection and measurement of the frequency of maximum oscillation power $ν_{max}$, without relying on the detection of a regular frequency spacing to guide the search. We applied the FRA pipeline to 254 synthetic power spectra representative of TESS red giants, as w… ▽ More We developed, tested and validated a new Fast, Robust and Automated (FRA) tool to detect solar-like oscillations. FRA is based on the detection and measurement of the frequency of maximum oscillation power $ν_{max}$, without relying on the detection of a regular frequency spacing to guide the search. We applied the FRA pipeline to 254 synthetic power spectra representative of TESS red giants, as well as 1689 red giants observed by Kepler and 2344 red giants observed by TESS. We obtain a consistency rate for $ν_{max}$ compared with existing measurements of $\sim$ 99% for Kepler red giants and of $\sim$ 98% for TESS red giants. We find that using $ν_{max}$ as an input parameter to guide the search for the large frequency separation $Δν$ through the existing Envelope AutoCorrelation Function (EACF) method significantly improves the consistency of the measured $Δν$ in the case of TESS stars, allowing to reach a consistency rate above 99%. Our analysis reveals that we can expect to get consistent $ν_{max}$ and $Δν$ measurements while minimizing both the false positive measurements and the non-detections for stars with a minimum of four observed sectors and a maximum G magnitude of 9.5. △ Less

Submitted 7 March, 2023; originally announced March 2023.

Comments: 35 pages, 17 figures. Accepted for publication in Astronomische Nachrichten. This is the pre-peer reviewed (submitted) version

Journal ref: Astronomische Nachrichten, Volume 344, Issue 5, article id. easna.20220090, 2023

TESS Asteroseismic Analysis of HD 76920: The Giant Star Hosting An Extremely Eccentric Exoplanet

Authors: Chen Jiang, Tao Wu, Adina D. Feinstein, Keivan G. Stassun, Timothy R. Bedding, Dimitri Veras, Enrico Corsaro, Derek L. Buzasi, Dennis Stello, Yaguang Li, Savita Mathur, Rafael A. Garcia, Sylvain N. Breton, Mia S. Lundkvist, Przemyslaw J. Mikolajczyk, Charlotte Gehan, Tiago L. Campante, Diego Bossini, Stephen R. Kane, Jia Mian Joel Ong, Mutlu Yildiz, Cenk Kayhan, Zeynep Celik Orhan, Sibel Ortel, Xinyi Zhang , et al. (8 additional authors not shown)

Abstract: The Transiting Exoplanet Survey Satellite (TESS) mission searches for new exoplanets. The observing strategy of TESS results in high-precision photometry of millions of stars across the sky, allowing for detailed asteroseismic studies of individual systems. In this work, we present a detailed asteroseismic analysis of the giant star HD 76920 hosting a highly eccentric giant planet ($e = 0.878$) wi… ▽ More The Transiting Exoplanet Survey Satellite (TESS) mission searches for new exoplanets. The observing strategy of TESS results in high-precision photometry of millions of stars across the sky, allowing for detailed asteroseismic studies of individual systems. In this work, we present a detailed asteroseismic analysis of the giant star HD 76920 hosting a highly eccentric giant planet ($e = 0.878$) with an orbital period of 415 days, using 5 sectors of TESS light curve that cover around 140 days of data. Solar-like oscillations in HD 76920 are detected around $52 \, μ$Hz by TESS for the first time. By utilizing asteroseismic modeling that takes classical observational parameters and stellar oscillation frequencies as constraints, we determine improved measurements of the stellar mass ($1.22 \pm 0.11\, M_\odot$), radius ($8.68 \pm 0.34\,R_\odot$), and age ($5.2 \pm 1.4\,$Gyr). With the updated parameters of the host star, we update the semi-major axis and mass of the planet as $a=1.165 \pm 0.035$ au and $M_{\rm p}\sin{i} = 3.57 \pm 0.22\,M_{\rm Jup}$. With an orbital pericenter of $0.142 \pm 0.005$ au, we confirm that the planet is currently far away enough from the star to experience negligible tidal decay until being engulfed in the stellar envelope. We also confirm that this event will occur within about 100\,Myr, depending on the stellar model used. △ Less

Submitted 6 February, 2023; v1 submitted 2 February, 2023; originally announced February 2023.

Comments: 18 pages, 7 figures, 4 tables

Evidence of structural discontinuities in the inner core of red-giant stars

Authors: Mathieu Vrard, Margarida S. Cunha, Diego Bossini, Pedro P. Avelino, Enrico Corsaro, Benoit Mosser

Abstract: Red giants are stars in the late stages of stellar evolution. Because they have exhausted the supply of hydrogen in their core, they burn the hydrogen in the surrounding shell. Once the helium in the core starts fusing, the star enters the clump phase, which is identified as a striking feature in the color-magnitude diagram. Since clump stars share similar observational properties, they are heavil… ▽ More Red giants are stars in the late stages of stellar evolution. Because they have exhausted the supply of hydrogen in their core, they burn the hydrogen in the surrounding shell. Once the helium in the core starts fusing, the star enters the clump phase, which is identified as a striking feature in the color-magnitude diagram. Since clump stars share similar observational properties, they are heavily used in astrophysical studies, as probes of distance, extinction through the galaxy, galaxy density, and stellar chemical evolution. In this work, we perform the detailed observational characterization of the deepest layers of clump stars using asteroseismic data from Kepler. We find evidence for large core structural discontinuities in about 6.7% of the stars in our sample, implying that the region of mixing beyond the convective core boundary has a radiative thermal stratification. These stars are otherwise similar to the remaining stars in our sample, which may indicate that the building of the discontinuities is an intermittent phenomenon. △ Less

Submitted 21 December, 2022; originally announced December 2022.

Comments: 30 pages, 4 figures, 1 table

Scalar particles around a Rindler-Schwarzschild wormhole

Authors: C. R. Muniz, H. R. Christiansen, M. S. Cunha, J. Furtado, V. B. Bezerra

Abstract: In this paper, we study quantum relativistic features of a scalar field around the Rindler-Schwarzschild wormhole. First, we introduce this new class of spacetime, investigating some energy conditions and verifying their violation in a region nearby the wormhole throat, which means that the object has to have an exotic energy in order to prevent its collapse. Then, we study the behavior of the mas… ▽ More In this paper, we study quantum relativistic features of a scalar field around the Rindler-Schwarzschild wormhole. First, we introduce this new class of spacetime, investigating some energy conditions and verifying their violation in a region nearby the wormhole throat, which means that the object has to have an exotic energy in order to prevent its collapse. Then, we study the behavior of the massless scalar field in this spacetime and compute the effective potential by means of tortoise coordinates. We show that such a potential is attractive nearby the wormhole throat and that is traversable via quantum tunneling by massive particles with sufficiently low energies. The solution of the Klein-Gordon equation is obtained subsequently, showing that the energy spectrum of the field is subject to a constraint which induces a decreasing oscillatory behavior. On imposing Dirichlet boundary conditions on a spherical shell nearby the throat we then determine the particle energy levels, and we use this spectrum to calculate the quantum revival of the eigenstates. Finally, we compute the Casimir energy associated with the massless scalar field at zero temperature. We perform this calculation by means of the sum of modes method. The zero-point energy is regularized using the Epstein-Hurwitz zeta-function. We also obtain an analytical expression for the Casimir force acting on the shell. △ Less

Submitted 13 October, 2022; originally announced October 2022.

Comments: 14 pages, 6 figures

Journal ref: Universe 2022, 8(12), 616

On quantum traversability of wormholes

Authors: J. Furtado, C. R. Muniz, M. S. Cunha, J. E. G. Silva

Abstract: In this paper we study the possibility of non-relativistic quantum particles to traverse the generalized Ellis-Bronnikov wormholes by considering quantum effects, such as tunneling. We have used the generalized Ellis-Bronnikov wormhole metric and found that for $n=2$ we have a single barrier shaped effective potential centered at the throat of the wormhole for any value of orbital angular momentum… ▽ More In this paper we study the possibility of non-relativistic quantum particles to traverse the generalized Ellis-Bronnikov wormholes by considering quantum effects, such as tunneling. We have used the generalized Ellis-Bronnikov wormhole metric and found that for $n=2$ we have a single barrier shaped effective potential centered at the throat of the wormhole for any value of orbital angular momentum. For $n\neq2$ we have a symmetric double barrier shaped potential when the orbital angular momentum is zero and a single barrier for nonzero angular orbital momentum. Analytical solutions for the Schrödinger equation in the generalized Ellis-Bronnikov spacetime could be found only for $n=2$. Such solutions were given in terms of the confluent Heun functions. Finally, by using a delta-barrier approximation we could find the transmission and reflection coefficients for a non-relativistic particle to traverse the generalized Ellis-Bronnikov wormhole. △ Less

Submitted 10 July, 2022; originally announced July 2022.

Comments: 5 pages, two columuns, 3 figures

Quasibound states of scalar fields in the consistent 4D Einstein-Gauss-Bonnet-(Anti-)de Sitter gravity

Authors: H. S. Vieira, V. B. Bezerra, C. R. Muniz, M. S. Cunha

Abstract: We examine the interaction between massless scalar fields and the gravitational field generated by a black hole solution that was recently obtained in the consistent well-defined 4-dimensional Einstein-Gauss-Bonnet gravity with a cosmological constant. In order to do this, we calculate quasibound state frequencies of scalar fields for the spherically symmetric black hole in the consistent 4-dimens… ▽ More We examine the interaction between massless scalar fields and the gravitational field generated by a black hole solution that was recently obtained in the consistent well-defined 4-dimensional Einstein-Gauss-Bonnet gravity with a cosmological constant. In order to do this, we calculate quasibound state frequencies of scalar fields for the spherically symmetric black hole in the consistent 4-dimensional Einstein-Gauss-Bonnet-de Sitter and Anti-de Sitter theories. The expression for the quasibound states is obtained by using the polynomial condition associated to the Heun functions, and their values are overdamped. We also demonstrate the stability of the systems. △ Less

Submitted 2 September, 2022; v1 submitted 31 May, 2022; originally announced May 2022.

Comments: 30 pages, 14 figures. arXiv admin note: text overlap with arXiv:2112.08711

Journal ref: Eur. Phys. J. C (2022) 82:669

Black strings from dark matter

Authors: Marcony S. Cunha, G. Alencar, Celio R. Muniz, Valdir B. Bezerra, Horácio S. Vieira

Abstract: In this paper, we obtain two different static black string solutions by considering as sources axisymmetric dark matter distributions in 3+1 dimensions. These solutions tend asymptotically to the usual static and uncharged black string vacuum solution predicted by General Relativity (GR). We show that both the solutions present an event horizon each, like the vacuum solution, which is larger than… ▽ More In this paper, we obtain two different static black string solutions by considering as sources axisymmetric dark matter distributions in 3+1 dimensions. These solutions tend asymptotically to the usual static and uncharged black string vacuum solution predicted by General Relativity (GR). We show that both the solutions present an event horizon each, like the vacuum solution, which is larger than the horizon of the latter. Then, we obtain the Hawking temperature associated with the black string solutions. Differently from what occurs with the static black string in the vacuum, we find that there exists a linear density of mass (or tension) remnant associated with a vanishing Hawking temperature for the obtained solutions. Thus, we analyze how the presence of dark matter affects the occurrence of the remnants. Further, we calculate other thermodynamic quantities, namely entropy, heat capacity, and free energy per length unit, showing that thermal phase transitions can occur in the presence of dark matter. We also analyze the weak (and null) energy conditions and conclude that the dark matter does not behave like an exotic fluid. Finally, we obtain the corresponding stationary solutions, determining their tensions as functions of both the mass and angular momentum of the black strings. △ Less

Submitted 6 April, 2023; v1 submitted 16 May, 2022; originally announced May 2022.

Comments: 16 pages. 9 figures

On the stellar core physics of the 16 Cyg binary system: constraining the central hydrogen abundance using asteroseismology

Authors: Benard Nsamba, Margarida S. Cunha, Catarina I. S. A. Rocha, Cristiano J. G. N. Pereira, Mário J. P. F. G. Monteiro, Tiago L. Campante

Abstract: The unprecedented quality of the asteroseismic data of solar-type stars made available by space missions such as NASA's Kepler telescope are making it possible to explore stellar interior structures. This offers possibilities of constraining stellar core properties (such as core sizes, abundances, and physics) paving the way for improving the precision of the inferred stellar ages. We employ 16 Cy… ▽ More The unprecedented quality of the asteroseismic data of solar-type stars made available by space missions such as NASA's Kepler telescope are making it possible to explore stellar interior structures. This offers possibilities of constraining stellar core properties (such as core sizes, abundances, and physics) paving the way for improving the precision of the inferred stellar ages. We employ 16 Cyg A and B as our benchmark stars for an asteroseismic study in which we present a novel approach aimed at selecting from a sample of acceptable stellar models returned from Forward Modelling techniques, down to the ones that better represent the core of each star. This is accomplished by comparing specific properties of the observed frequency ratios for each star to the ones derived from the acceptable stellar models. We demonstrate that in this way we are able to constrain further the hydrogen mass fraction in the core, establishing the stars' precise evolutionary states and ages. The ranges of the derived core hydrogen mass fractions are [0.01 - 0.06] and [0.12 - 0.19] for 16 Cyg A and B, respectively, and, considering that the stars are coeval, the age and metal mass fraction parameters span the region [6.4 - 7.4] Gyr and [0.023 - 0.026], respectively. In addition, our findings show that using a single helium-to-heavy element enrichment ratio, ($ΔY/ΔZ$), when forward modelling the 16 Cyg binary system, may result in a sample of acceptable models that do not simultaneously fit the observed frequency ratios, further highlighting that such an approach to the definition of the helium content of the star may not be adequate in studies of individual stars. △ Less

Submitted 10 May, 2022; originally announced May 2022.

Comments: 13 pages, 8 figures, and 5 tables. Accepted for publication in MNRAS

Four-dimensional regular black strings in bilocal gravity

Authors: C. R. Muniz, H. R. Christiansen, M. S. Cunha, J. Furtado

Abstract: In this paper, we obtain a static black string solution for a bilocal gravitational source in 3+1 dimensions. The solution is regular at the origin and tends asymptotically to the ordinary static uncharged black string solution of general relativity. It allows an event horizon and an internal horizon depending on the value of the mass density. A mass remnant associated with a vanishing Hawking tem… ▽ More In this paper, we obtain a static black string solution for a bilocal gravitational source in 3+1 dimensions. The solution is regular at the origin and tends asymptotically to the ordinary static uncharged black string solution of general relativity. It allows an event horizon and an internal horizon depending on the value of the mass density. A mass remnant associated with a vanishing Hawking temperature is also found. In order to stabilize the solution, a perfect cosmological fluid with state parameter $ω>-1$ should be present throughout space. However, energy conditions do not exclude an exotic substance nearby the black string. Finally, we find the stationary counterpart of the solution and analyze the behavior of the horizons according to the mass and angular momentum of the spinning object. △ Less

Submitted 30 June, 2022; v1 submitted 28 February, 2022; originally announced February 2022.

Comments: 12 pages, 5 figures, text improved, final version to be published in the Annals of Physics

Journal ref: Annals of Physics 443, 168980 (2022)

New regular 2+1 black hole solutions from bilocal gravity

Authors: H. R. Christiansen, Milko Estrada, M. S. Cunha, J. Furtado, C. R. Muniz

Abstract: We obtain new regular black hole solutions for an action in 2+1 dimensions with bilocal Ricci scalar and negative cosmological constant. Besides their connection to the cosmological constant, these solutions depend on a fundamental length due to their non-local nature. The effective profile densities that result from the non-local geometries have quasi-localized mass/energy since they are finite a… ▽ More We obtain new regular black hole solutions for an action in 2+1 dimensions with bilocal Ricci scalar and negative cosmological constant. Besides their connection to the cosmological constant, these solutions depend on a fundamental length due to their non-local nature. The effective profile densities that result from the non-local geometries have quasi-localized mass/energy since they are finite at the origin and their integration in all space is convergent. The black holes obtained are free of singularities and present one, two, or none horizons depending on the values of the involved parameters. The new solutions can have either an AdS, dS, or even a flat core. In the case of a de-Sitter core, it could represent a repulsive force coming from quantum effects. Although the resulting (effective) cosmological constant is positive near the origin, the classical (naked) counterpart is still negative thus precluding a cosmological horizon. We investigate the energy conditions of the effective source and determine the region where exotic energy should be found. Thermodynamic quantities are also computed. On the one hand, Gibbs's potential shows that both solutions are globally unstable, as in the BTZ case. On the other, we show that for small values of the horizon radius the Hawking temperature is negatively divergent but a finite size remnant can be defined where $T_H$ crosses zero. At this point, the heat capacity sign changes from negative to positive, indicating that the black holes are locally stable while irradiating. Thus, such a quantity, along with $T_H$, presents crucial differences with the BTZ black hole for small horizon radii where quantum effects become relevant. Finally, we analyze the bilocal black hole geodesics and find stable circular orbits for massless and massive particles, another feature absent in the BTZ case. △ Less

Submitted 6 April, 2023; v1 submitted 30 January, 2022; originally announced January 2022.

Comments: Text improved, 20 pages, 7 figures. Accepted for publication in Int. J. Mod. Phys. D

PLATO Hare-and-Hounds exercise: Asteroseismic model fitting of main-sequence solar-like pulsators

Authors: M. S. Cunha, I. W. Roxburgh, V. Aguirre Børsen-Koch, W. H. Ball, S. Basu, W. J. Chaplin, M. -J. Goupil, B. Nsamba, J. Ong, D. R. Reese, K. Verma, K. Belkacem, T. Campante, J. Christensen-Dalsgaard, M. T. Clara, S. Deheuvels, M. J. P. F. G. Monteiro, A. Noll, R. M. Ouazzani, J. L. Rørsted, A. Stokholm, M. L. Winther

Abstract: Asteroseismology is a powerful tool to infer fundamental stellar properties. The use of these asteroseismic-inferred properties in a growing number of astrophysical contexts makes it vital to understand their accuracy. Consequently, we performed a hare-and-hounds exercise where the hares simulated data for 6 artificial main-sequence stars and the hounds inferred their properties based on different… ▽ More Asteroseismology is a powerful tool to infer fundamental stellar properties. The use of these asteroseismic-inferred properties in a growing number of astrophysical contexts makes it vital to understand their accuracy. Consequently, we performed a hare-and-hounds exercise where the hares simulated data for 6 artificial main-sequence stars and the hounds inferred their properties based on different inference procedures. To mimic a pipeline such as that planned for the PLATO mission, all hounds used the same model grid. Some stars were simulated using the physics adopted in the grid, others a different one. The maximum relative differences found (in absolute value) between the inferred and true values of the mass, radius, and age were 4.32 per cent, 1.33 per cent, and 11.25 per cent, respectively. The largest systematic differences in radius and age were found for a star simulated assuming gravitational settling, not accounted for in the model grid, with biases of -0.88 per cent (radius) and 8.66 per cent (age). For the mass, the most significant bias (-3.16 per cent) was found for a star with a helium enrichment ratio outside the grid range. Moreover, a ~7 per cent dispersion in age was found when adopting different prescriptions for the surface corrections or shifting the classical observations by $\pm 1σ$. The choice of the relative weight given to the classical and seismic constraints also impacted significantly the accuracy and precision of the results. Interestingly, only a few frequencies were required to achieve accurate results on the mass and radius. For the age the same was true when at least one $l=2$ mode was considered. △ Less

Submitted 7 October, 2021; originally announced October 2021.

Comments: Accepted for publication in the Monthly Notices of the Royal Astronomical Society

Diagnostics from solar and stellar glitches

Authors: Margarida S. Cunha

Abstract: Sudden changes in the internal structure of stars, placed at the interface between convective and radiative regions, regions of partial ionisation, or between layers that have acquired different chemical composition as a result of nuclear burning, often produce specific signatures in the stars oscillation spectra. Through the study of these signatures one may gain information on the physical proce… ▽ More Sudden changes in the internal structure of stars, placed at the interface between convective and radiative regions, regions of partial ionisation, or between layers that have acquired different chemical composition as a result of nuclear burning, often produce specific signatures in the stars oscillation spectra. Through the study of these signatures one may gain information on the physical processes that shape the regions that produce them, including diffusion and chemical mixing beyond the convectively unstable regions, as well as information about the helium content of stars. In this talk, I will review important theoretical and observational efforts conducted over the years towards this goal. I will emphasise the potential offered by the study of acoustic, gravity, and mixed modes observed in stars of different mass and evolutionary stages, at a time when space-based data is allowing us to build on the knowledge gained from the study of the sun and white dwarfs, where these efforts have long been undertaken, extending the methods developed to stars across the HR diagramme. △ Less

Submitted 8 September, 2021; originally announced September 2021.

Comments: Review presented at the conference: Dynamics of the Sun and Stars (Honouring the Life and Work of Michael J. Thompson)

Journal ref: In: Monteiro M.J.P.F.G., García R.A., Christensen-Dalsgaard J., McIntosh S.W. (eds) Dynamics of the Sun and Stars. Astrophysics and Space Science Proceedings, vol 57. Springer (2020)

A 20-Second Cadence View of Solar-Type Stars and Their Planets with TESS: Asteroseismology of Solar Analogs and a Re-characterization of pi Men c

Authors: Daniel Huber, Timothy R. White, Travis S. Metcalfe, Ashley Chontos, Michael M. Fausnaugh, Cynthia S. K. Ho, Vincent Van Eylen, Warrick Ball, Sarbani Basu, Timothy R. Bedding, Othman Benomar, Diego Bossini, Sylvain Breton, Derek L. Buzasi, Tiago L. Campante, William J. Chaplin, Joergen Christensen-Dalsgaard, Margarida S. Cunha, Morgan Deal, Rafael A. Garcia, Antonio Garcia Munoz, Charlotte Gehan, Lucia Gonzalez-Cuesta, Chen Jiang, Cenk Kayhan , et al. (28 additional authors not shown)

Abstract: We present an analysis of the first 20-second cadence light curves obtained by the TESS space telescope during its extended mission. We find a precision improvement of 20-second data compared to 2-minute data for bright stars when binned to the same cadence (~10-25% better for T<~8 mag, reaching equal precision at T~13 mag), consistent with pre-flight expectations based on differences in cosmic ra… ▽ More We present an analysis of the first 20-second cadence light curves obtained by the TESS space telescope during its extended mission. We find a precision improvement of 20-second data compared to 2-minute data for bright stars when binned to the same cadence (~10-25% better for T<~8 mag, reaching equal precision at T~13 mag), consistent with pre-flight expectations based on differences in cosmic ray mitigation algorithms. We present two results enabled by this improvement. First, we use 20-second data to detect oscillations in three solar analogs (gamma Pav, zeta Tuc and pi Men) and use asteroseismology to measure their radii, masses, densities and ages to ~1%, ~3%, ~1% and ~20% respectively, including systematic errors. Combining our asteroseismic ages with chromospheric activity measurements we find evidence that the spread in the activity-age relation is linked to stellar mass and thus convection-zone depth. Second, we combine 20-second data and published radial velocities to re-characterize pi Men c, which is now the closest transiting exoplanet for which detailed asteroseismology of the host star is possible. We show that pi Men c is located at the upper edge of the planet radius valley for its orbital period, confirming that it has likely retained a volatile atmosphere and that the "asteroseismic radius valley" remains devoid of planets. Our analysis favors a low eccentricity for pi Men c (<0.1 at 68% confidence), suggesting efficient tidal dissipation (Q/k <~ 2400) if it formed via high-eccentricity migration. Combined, these early results demonstrate the strong potential of TESS 20-second cadence data for stellar astrophysics and exoplanet science. △ Less

Submitted 13 October, 2021; v1 submitted 20 August, 2021; originally announced August 2021.

Comments: 17 pages (excluding references), 13 figures, 6 tables; accepted for publication in AJ. Data and scripts to reproduce results are archived at https://zenodo.org/record/5555456

On the relation between active-region lifetimes and the autocorrelation function of light curves

Authors: A. R. G. Santos, S. Mathur, R. A. García, M. S. Cunha, P. P. Avelino

Abstract: Rotational modulation of stellar light curves due to dark spots encloses information on spot properties and, thus, on magnetic activity. In particular, the decay of the autocorrelation function (ACF) of light curves is presumed to be linked to spot/active-region lifetimes, given that some coherence of the signal is expected throughout their lifetime. In the literature, an exponential decay has bee… ▽ More Rotational modulation of stellar light curves due to dark spots encloses information on spot properties and, thus, on magnetic activity. In particular, the decay of the autocorrelation function (ACF) of light curves is presumed to be linked to spot/active-region lifetimes, given that some coherence of the signal is expected throughout their lifetime. In the literature, an exponential decay has been adopted to describe the ACF. Here, we investigate the relation between the ACF and the active-region lifetimes. For this purpose, we produce artificial light curves of rotating spotted stars with different observation, stellar, and spot properties. We find that a linear decay and respective timescale better represent the ACF than the exponential decay. We therefore adopt a linear decay. The spot/active-region timescale inferred from the ACF is strongly restricted by the observation length of the light curves. For 1-year light curves our results are consistent with no correlation between the inferred and the input timescales. The ACF decay is also significantly affected by differential rotation and spot evolution: strong differential rotation and fast spot evolution contribute to a more severe underestimation of the active-region lifetimes. Nevertheless, in both circumstances the observed timescale is still correlated with the input lifetimes. Therefore, our analysis suggests that the ACF decay can be used to obtain a lower limit of the active-region lifetimes for relatively long-term observations. However, strategies to avoid or flag targets with fast active-region evolution or displaying stable beating patterns associated with differential rotation should be employed. △ Less

Submitted 19 August, 2021; originally announced August 2021.

Comments: 12 pages, 19 Figures, Accepted for publication in MNRAS

Asteroseismology of iota Draconis and Discovery of an Additional Long-Period Companion

Authors: Michelle L. Hill, Stephen R. Kane, Tiago L. Campante, Zhexing Li, Paul A. Dalba, Timothy D. Brandt, Timothy R. White, Benjamin J. S. Pope, Keivan G. Stassun, Benjamin J. Fulton, Enrico Corsaro, Tanda Li, J. M. Joel Ong, Timothy R. Bedding, Diego Bossini, Derek L. Buzasi, William J. Chaplin, Margarida S. Cunha, Rafael A. Garcia, Sylvain N. Breton, Marc Hon, Daniel Huber, Chen Jiang, Cenk Kayhan, James S. Kuszlewicz , et al. (3 additional authors not shown)

Abstract: Giant stars as known exoplanet hosts are relatively rare due to the potential challenges in acquiring precision radial velocities and the small predicted transit depths. However, these giant host stars are also some of the brightest in the sky and so enable high signal-to-noise follow-up measurements. Here we report on new observations of the bright (V ~ 3.3) giant star $ι$ Draconis ($ι$ Dra), kno… ▽ More Giant stars as known exoplanet hosts are relatively rare due to the potential challenges in acquiring precision radial velocities and the small predicted transit depths. However, these giant host stars are also some of the brightest in the sky and so enable high signal-to-noise follow-up measurements. Here we report on new observations of the bright (V ~ 3.3) giant star $ι$ Draconis ($ι$ Dra), known to host a planet in a highly eccentric ~511 day period orbit. TESS observations of the star over 137 days reveal asteroseismic signatures, allowing us to constrain the stellar radius, mass, and age to ~2%, ~6%, and ~28%, respectively. We present the results of continued radial velocity monitoring of the star using the Automated Planet Finder over several orbits of the planet. We provide more precise planet parameters of the known planet and, through the combination of our radial velocity measurements with Hipparcos and Gaia astrometry, we discover an additional long-period companion with an orbital period of ~$68^{+60}_{-36}$ years. Mass predictions from our analysis place this sub-stellar companion on the border of the planet and brown dwarf regimes. The bright nature of the star combined with the revised orbital architecture of the system provides an opportunity to study planetary orbital dynamics that evolve as the star moves into the giant phase of its evolution. △ Less

Submitted 28 July, 2021; originally announced July 2021.

Comments: 20 pages, 11 Figures, Accepted for publication in the Astronomical Journal

Discovery of multiple p-mode pulsation frequencies in the roAp star, HD 86181

Authors: Fangfei Shi, Donald W. Kurtz, Daniel L. Holdsworth, Hideyuki Saio, Margarida S. Cunha, Huawei Zhang, Jianning Fu, G. Handler

Abstract: We report the frequency analysis of a known roAp star, HD 86181 (TIC 469246567), with new inferences from TESS data. We derive the rotation frequency to be $ν_{rot}$ = 0.48753 $\pm$ 0.00001d$^{-1}$. The pulsation frequency spectrum is rich, consisting of two doublets and one quintuplet, which we interpret to be oblique pulsation multiplets from consecutive, high-overtone dipole, quadrupole and dip… ▽ More We report the frequency analysis of a known roAp star, HD 86181 (TIC 469246567), with new inferences from TESS data. We derive the rotation frequency to be $ν_{rot}$ = 0.48753 $\pm$ 0.00001d$^{-1}$. The pulsation frequency spectrum is rich, consisting of two doublets and one quintuplet, which we interpret to be oblique pulsation multiplets from consecutive, high-overtone dipole, quadrupole and dipole modes. The central frequency of the quintuplet is 232.7701d$^{-1}$ (2.694 mHz). The phases of the sidelobes, the pulsation phase modulation, and a spherical harmonic decomposition all show that the quadrupole mode is distorted. Following the oblique pulsator model, we calculate the rotation inclination, i, and magnetic obliquity, $β$, of this star, which provide detailed information about the pulsation geometry. The i and $β$ derived from the best fit of the pulsation amplitude and phase modulation to a theoretical model, including the magnetic field effect, slightly differ from those calculated for a pure quadrupole, indicating the contributions from l = 4, 6, 8, ... are small. Non-adiabatic models with different envelope convection conditions and physics configurations were considered for this star. It is shown that models with envelope convection almost fully suppressed can explain the excitation at the observed pulsation frequencies. △ Less

Submitted 26 July, 2021; originally announced July 2021.

TESS Cycle 1 observations of roAp stars with 2-min cadence data

Authors: D. L. Holdsworth, M. S. Cunha, D. W. Kurtz, V. Antoci, D. R. Hey, D. M. Bowman, O. Kobzar, D. L. Buzasi, O. Kochukhov, E. Niemczura, D. Ozuyar, F. Shi, R. Szabó, A. Samadi-Ghadim, Zs. Bognár, L. Fox-Machado, V. Khalack, M. Lares-Martiz, C. C. Lovekin, P. Mikołajczyk, D. Mkrtichian, J. Pascual-Granado, E. Paunzen, T. Richey-Yowell, Á. Sódor , et al. (19 additional authors not shown)

Abstract: We present the results of a systematic search for new rapidly oscillating Ap (roAp) stars using the 2-min cadence data collected by the Transiting Exoplanet Survey Satellite (TESS) during its Cycle 1 observations. We identify 12 new roAp stars. Amongst these stars we discover the roAp star with the longest pulsation period, another with the shortest rotation period, and six with multiperiodic vari… ▽ More We present the results of a systematic search for new rapidly oscillating Ap (roAp) stars using the 2-min cadence data collected by the Transiting Exoplanet Survey Satellite (TESS) during its Cycle 1 observations. We identify 12 new roAp stars. Amongst these stars we discover the roAp star with the longest pulsation period, another with the shortest rotation period, and six with multiperiodic variability. In addition to these new roAp stars, we present an analysis of 44 known roAp stars observed by TESS during Cycle 1, providing the first high-precision and homogeneous sample of a significant fraction of the known roAp stars. The TESS observations have shown that almost 60 per cent (33) of our sample of stars are multiperiodic, providing excellent cases to test models of roAp pulsations, and from which the most rewarding asteroseismic results can be gleaned. We report four cases of the occurrence of rotationally split frequency multiplets that imply different mode geometries for the same degree modes in the same star. This provides a conundrum in applying the oblique pulsator model to the roAp stars. Finally, we report the discovery of non-linear mode interactions in $α$ Cir (TIC 402546736, HD 128898) around the harmonic of the principal mode -- this is only the second case of such a phenomenon. △ Less

Submitted 27 May, 2021; originally announced May 2021.

Comments: Accepted for publication in MNRAS. 38 Pages, 50 Figures, 1 Table, 2 Appendices

Fundamental properties of a selected sample of Ap stars: Inferences from interferometric and asteroseismic constraints

Authors: M. Deal, M. S. Cunha, Z. Keszthelyi, K. Perraut, D. L. Holdsworth

Abstract: Magnetic fields influence the formation and evolution of stars and impact the observed stellar properties. Ap stars (magnetic A-type stars) are a prime example of this. Access to precise and accurate determinations of their stellar fundamental properties, such as masses and ages, is crucial to understand the origin and evolution of fossil magnetic fields. We propose using the radii and luminositie… ▽ More Magnetic fields influence the formation and evolution of stars and impact the observed stellar properties. Ap stars (magnetic A-type stars) are a prime example of this. Access to precise and accurate determinations of their stellar fundamental properties, such as masses and ages, is crucial to understand the origin and evolution of fossil magnetic fields. We propose using the radii and luminosities determined from interferometric measurements, in addition to seismic constraints when available, to infer fundamental properties of 14 Ap stars préviously characterised. We used a grid-based modelling approach, employing stellar models computed with the \textsc{cestam} stellar evolution code, and the parameter search performed with the \textsc{aims} optimisation method. The stellar model grid was built using a wide range of initial helium abundances and metallicities in order to avoid any bias originating from the initial chemical composition. The large frequency separations ($Δν$) of HR\,1217 (HD\,24712) and $α$~Cir (HD\,128898), two rapidly oscillating Ap stars of the sample, were used as seismic constraints. We inferred the fundamental properties of the 14 stars in the sample. The overall results are consistent within $1σ$ with previous studies, however, the stellar masses inferred in this study are higher. This trend likely originates from the broader range of chemical compositions considered in this work. We show that the use of $Δν$ in the modelling significantly improves our inferences, allowing us to set reasonable constraints on the initial metallicity which is, otherwise, unconstrained. This gives an indication of the efficiency of atomic diffusion in the atmospheres of roAp stars and opens the possibility of characterising the transport of chemical elements in their interiors. △ Less

Submitted 16 April, 2021; originally announced April 2021.

Comments: 19 pages, 14 figures, accepted for publication in A&A

Journal ref: A&A 650, A125 (2021)

Automated approach to measure stellar inclinations: validation through large-scale measurements on the red giant branch

Authors: C. Gehan, B. Mosser, E. Michel, M. S. Cunha

Abstract: Measuring stellar inclinations is fundamental to understand planetary formation and dynamics as well as physical conditions during star formation. Oscillation spectra of red giant stars exhibit mixed modes that have both a gravity component from the radiative interior and a pressure component from the convective envelope. Gravity-dominated (g-m) mixed modes split by rotation are well separated ins… ▽ More Measuring stellar inclinations is fundamental to understand planetary formation and dynamics as well as physical conditions during star formation. Oscillation spectra of red giant stars exhibit mixed modes that have both a gravity component from the radiative interior and a pressure component from the convective envelope. Gravity-dominated (g-m) mixed modes split by rotation are well separated inside frequency spectra, making possible accurate measurements of stellar inclinations. This work aims at developing an automated and general approach to measure stellar inclinations, that can be applied to any solar-type pulsator for which oscillation modes are identified, and at validating it using red giant branch stars observed by Kepler. We use the mean height-to-background ratio of dipole mixed modes with different azimuthal orders to measure stellar inclinations. The underlying statistical distribution of inclinations is recovered in an unbiased way using a probability density function for the stellar inclination angle. We derive stellar inclination measurements for 1139 stars on the red giant branch, for which Gehan et al. (2018) have identified the azimuthal order of dipole g-m mixed modes. Raw measured inclinations exhibit strong deviation with respect to isotropy which is expected for random inclinations over the sky. When taking uncertainties into account, the reconstructed distribution of inclinations actually follows the expected isotropic distribution of the rotational axis. This work highlights the biases that affect inclination measurements and provides the way to infer their underlying statistical distribution. When the star is seen either pole-on or equator-on, measurements are challenging and result in a biased distribution. Correcting biases that appear at the low- and high inclination regimes allows us to recover the underlying inclination distribution. △ Less

Submitted 20 November, 2020; v1 submitted 4 November, 2020; originally announced November 2020.

Comments: 35 pages, accepted for publication in A&A

Journal ref: A&A 645, A124 (2021)

Asteroseismic modelling of solar-type stars: A deeper look at the treatment of initial helium abundance

Authors: Benard Nsamba, Nuno Moedas, Tiago L. Campante, Margarida S. Cunha, Antonio García Hernández, Juan C. Suárez, Mário J. P. F. G. Monteiro, João Fernandes, Chen Jiang, Babatunde Akinsanmi

Abstract: Detailed understanding of stellar physics is essential towards a robust determination of stellar properties (e.g. radius, mass, and age). Among the vital input physics used in the modelling of solar-type stars which remain poorly constrained, is the initial helium abundance. To this end, when constructing stellar model grids, the initial helium abundance is estimated either (i) by using the semi-e… ▽ More Detailed understanding of stellar physics is essential towards a robust determination of stellar properties (e.g. radius, mass, and age). Among the vital input physics used in the modelling of solar-type stars which remain poorly constrained, is the initial helium abundance. To this end, when constructing stellar model grids, the initial helium abundance is estimated either (i) by using the semi-empirical helium-to-heavy element enrichment ratio, (${ΔY}/{ΔZ}$), anchored to the standard Big Bang Nucleosynthesis value or (ii) by setting the initial helium abundance as a free variable. Adopting 35 low-mass, solar-type stars with multi-year Kepler photometry from the asteroseismic "LEGACY" sample, we explore the systematic uncertainties on the inferred stellar parameters (i.e., radius, mass, and age) arising from the treatment of the initial helium abundance in stellar model grids . The stellar masses and radii derived from grids with free initial helium abundance are lower compared to those from grids based on a fixed ${ΔY}/{ΔZ}$ ratio. We find the systematic uncertainties on mean density, radius, mass, and age arising from grids which employ a fixed value of ${ΔY}/{ΔZ}$ and those with free initial helium abundance to be $\sim$ 0.9%, $\sim$ 2%, $\sim$ 5% and $\sim$ 29%, respectively. We report that the systematic uncertainties on the inferred masses and radii arising from the treatment of initial helium abundance in stellar grids lie within the expected accuracy limits of ESA's PLATO, although this is not the case for the age. △ Less

Submitted 15 October, 2020; originally announced October 2020.

Comments: 12 pages, 10 figures, and 6 tables. Accepted for publication in MNRAS

Robust asteroseismic properties of the bright planet host HD 38529

Authors: Warrick H. Ball, William J. Chaplin, Martin B. Nielsen, Lucia González-Cuesta, Savita Mathur, Ângela R. G. Santos, Rafael García, Derek Buzasi, Benoît Mosser, Morgan Deal, Amalie Stokholm, Jakob Rørsted Mosumgaard, Victor Silva Aguirre, Benard Nsamba, Tiago Campante, Margarida S. Cunha, Joel Ong, Sarbani Basu, Sibel Örtel, Z. Çelik Orhan, Mutlu Yıldız, Keivan Stassun, Stephen R. Kane, Daniel Huber

Abstract: The Transiting Exoplanet Survey Satellite (TESS) is recording short-cadence, high duty-cycle timeseries across most of the sky, which presents the opportunity to detect and study oscillations in interesting stars, in particular planet hosts. We have detected and analysed solar-like oscillations in the bright G4 subgiant HD 38529, which hosts an inner, roughly Jupiter-mass planet on a 14.3 d orbit… ▽ More The Transiting Exoplanet Survey Satellite (TESS) is recording short-cadence, high duty-cycle timeseries across most of the sky, which presents the opportunity to detect and study oscillations in interesting stars, in particular planet hosts. We have detected and analysed solar-like oscillations in the bright G4 subgiant HD 38529, which hosts an inner, roughly Jupiter-mass planet on a 14.3 d orbit and an outer, low-mass brown dwarf on a 2136 d orbit. We combine results from multiple stellar modelling teams to produce robust asteroseismic estimates of the star's properties, including its mass $M = 1.48 \pm 0.04 \mathrm{M}_\odot$, radius $R = 2.68 \pm 0.03 \mathrm{R}_\odot$ and age $t = 3.07 \pm 0.39 \,\mathrm{Gyr}$. Our results confirm that HD 38529 has a mass near the higher end of the range that can be found in the literature and also demonstrate that precise stellar properties can be measured given shorter timeseries than produced by CoRoT, Kepler or K2. △ Less

Submitted 14 October, 2020; originally announced October 2020.

Comments: 10 pages, 6 figures, 4 tables, accepted for publication in MNRAS

From Solar-like to Mira stars: a unifying description of stellar pulsators in the presence of stochastic noise

Authors: Margarida S. Cunha, Pedro P. Avelino, William J. Chaplin

Abstract: We discuss and characterise the power spectral density properties of a model aimed at describing pulsations in stars from the main-sequence to the asymptotic giant branch. We show that the predicted limit of the power spectral density for a pulsation mode in the presence of stochastic noise is always well approximated by a Lorentzian function. While in stars predominantly stochastically driven the… ▽ More We discuss and characterise the power spectral density properties of a model aimed at describing pulsations in stars from the main-sequence to the asymptotic giant branch. We show that the predicted limit of the power spectral density for a pulsation mode in the presence of stochastic noise is always well approximated by a Lorentzian function. While in stars predominantly stochastically driven the width of the Lorentzian is defined by the mode lifetime, in stars where the driving is predominately coherent the width is defined by the amplitude of the stochastic perturbations. In stars where both drivings are comparable, the width is defined by both these parameters and is smaller than that expected from pure stochastic driving. We illustrate our model through numerical simulations and propose a well defined classification of stars into predominantly stochastic (solar-like) and predominately coherent (classic) pulsators. We apply the model to the study of the Mira variable U Per, and the semiregular variable L2 Pup and, following our classification, conclude that they are both classical pulsators. Our model provides a natural explanation for the change in behaviour of the pulsation amplitude-period relation noted in several earlier works. Moreover, our study of L2 Pup enables us to test the scaling relation between the mode line width and effective temperature, confirming that an exponential scaling reproduces well the data all the way from the main sequence to the asymptotic giant branch, down to temperatures about 1000 K below what has been tested in previous studies. △ Less

Submitted 21 September, 2020; originally announced September 2020.

Comments: 11 pages, 8 figures, accepted for publication in MNRAS

On using dipolar modes to constrain the helium glitch in red-giant stars

Authors: G. Dréau, M. S. Cunha, M. Vrard, P. P. Avelino

Abstract: The space-borne missions CoRoT and Kepler have revealed numerous mixed modes in red-giant stars. These modes carry a wealth of information about red-giant cores, but are of limited use when constraining rapid structural variations in their envelopes. This limitation can be circumvented if we have access to the frequencies of the pure acoustic dipolar modes in red giants, i.e. the dipole modes that… ▽ More The space-borne missions CoRoT and Kepler have revealed numerous mixed modes in red-giant stars. These modes carry a wealth of information about red-giant cores, but are of limited use when constraining rapid structural variations in their envelopes. This limitation can be circumvented if we have access to the frequencies of the pure acoustic dipolar modes in red giants, i.e. the dipole modes that would exist in the absence of coupling between gravity and acoustic waves. We present a pilot study aimed at evaluating the implications of using these pure acoustic mode frequencies in seismic studies of the helium structural variation in red giants. The study is based on artificial seismic data for a red-giant-branch stellar model, bracketing seven acoustic dipole radial orders around vmax. The pure acoustic dipole-mode frequencies are derived from a fit to the mixed-mode period spacings and then used to compute the pure acoustic dipole-mode second differences. The pure acoustic dipole-mode second differences inferred through this procedure follow the same oscillatory function as the radial modes second differences. The additional constraints brought by the dipolar modes allow us to adopt a more complete description of the glitch signature when performing the fit to the second differences. The amplitude of the glitch retrieved from this fit is 15% smaller than that from the fit based on the radial modes alone. Also, we find that thanks to the additional constraints, a bias in the inferred glitch location, found when adopting the simpler description of the glitch, is avoided. △ Less

Submitted 3 July, 2020; originally announced July 2020.

Comments: 7 pages, 4 figures, accepted for publication in the MNRAS

TESS Asteroseismic Analysis of the Known Exoplanet Host Star HD 222076

Authors: Chen Jiang, Timothy R. Bedding, Keivan G. Stassun, Dimitri Veras, Enrico Corsaro, Derek L. Buzasi, Przemysław Mikołajczyk, Qian-sheng, Zhang, Jian-wen, Ou, Tiago L. Campante, Thaíse S. Rodrigues, Benard Nsamba, Diego Bossini, Stephen R. Kane, Jia Mian Joel Ong, Mutlu Yıldız, Zeynep Çeiik Orhan, Sibel Örtel, Tao Wu, Xinyi Zhang, Tanda Li, Sarbani Basu, Margarida S. Cunha , et al. (2 additional authors not shown)

Abstract: The Transiting Exoplanet Survey Satellite (TESS) is an all-sky survey mission aiming to search for exoplanets that transit bright stars. The high-quality photometric data of TESS are excellent for the asteroseismic study of solar-like stars. In this work, we present an asteroseismic analysis of the red-giant star HD~222076 hosting a long-period (2.4 yr) giant planet discovered through radial veloc… ▽ More The Transiting Exoplanet Survey Satellite (TESS) is an all-sky survey mission aiming to search for exoplanets that transit bright stars. The high-quality photometric data of TESS are excellent for the asteroseismic study of solar-like stars. In this work, we present an asteroseismic analysis of the red-giant star HD~222076 hosting a long-period (2.4 yr) giant planet discovered through radial velocities. Solar-like oscillations of HD~222076 are detected around $203 \, μ$Hz by TESS for the first time. Asteroseismic modeling, using global asteroseismic parameters as input, yields a determination of the stellar mass ($M_\star = 1.12 \pm 0.12\, M_\odot$), radius ($R_\star = 4.34 \pm 0.21\,R_\odot$), and age ($7.4 \pm 2.7\,$Gyr), with precisions greatly improved from previous studies. The period spacing of the dipolar mixed modes extracted from the observed power spectrum reveals that the star is on the red-giant branch burning hydrogen in a shell surrounding the core. We find that the planet will not escape the tidal pull of the star and be engulfed into it within about $800\,$Myr, before the tip of the red-giant branch is reached. △ Less

Submitted 1 May, 2020; originally announced May 2020.

Comments: 18 pages, 6 figures, 2 tables

Class of solutions of the Wheeler-DeWitt equation with ordering parameter

Authors: H. S. Vieira, V. B. Bezerra, C. R. Muniz, M. S. Cunha, H. R. Christiansen

Abstract: In this letter, we discuss the Wheeler-DeWitt equation with an ordering parameter in the Friedmann-Robertson-Walker universe. The solutions when the universe was very small and at the end of the expansion are obtained in terms of Bessel and Heun functions, respectively. We also obtain a boundary condition which should be satisfied by the ordering parameter, namely, $0 \leq p \leq 2$. We investigat… ▽ More In this letter, we discuss the Wheeler-DeWitt equation with an ordering parameter in the Friedmann-Robertson-Walker universe. The solutions when the universe was very small and at the end of the expansion are obtained in terms of Bessel and Heun functions, respectively. We also obtain a boundary condition which should be satisfied by the ordering parameter, namely, $0 \leq p \leq 2$. We investigate the minimum value of the scale factor with respect to the maximum value of the probability density. △ Less

Submitted 27 August, 2020; v1 submitted 30 March, 2020; originally announced March 2020.

Comments: 6 pages, 9 figures

MSC Class: 81Q05; 83C45; 83C57; 83C75

Journal ref: Physics Letters B 809 (2020) 135712