Showing 1–50 of 158 results for author: Baraffe, I

Revealing mixed modes in compressible hydrodynamical simulations of red giant stars

Authors: Nils B. de Vries, Arthur Le Saux, Isabelle Baraffe, Thomas Guillet, Richard H. D. Townsend, Armand Leclerc, Adrien Morison

Abstract: Mixed modes are observed in many low-mass evolved stars. They provide information about core rotation rates of these stars, which are lower than predicted by stellar evolution models. The mixed modes themselves have been invoked as an angular momentum transport mechanism, but estimating their transport efficiency requires knowledge of their amplitudes. We constrain, for the first time, the mixed m… ▽ More Mixed modes are observed in many low-mass evolved stars. They provide information about core rotation rates of these stars, which are lower than predicted by stellar evolution models. The mixed modes themselves have been invoked as an angular momentum transport mechanism, but estimating their transport efficiency requires knowledge of their amplitudes. We constrain, for the first time, the mixed mode amplitudes in 2D hydrodynamical simulations of a $1.3M_\odot$ red giant using the code \textsc{music}. We perform two simulations with outer radial truncations at fractional radii $r_o/r_\star = 0.90$ and $r_o/r_\star = 0.98$. We compare the modes in the simulation with those found using both \textsc{gyre} and a \textsc{dedalus} eigenvalue solver. Excellent frequency agreement is found for all p-dominated modes, with minor discrepancies for g-dominated modes, especially in the frequency range $[60, \ 240]\ μ\mathrm{Hz}$. We find excellent eigenfunction agreement for all modes except those in this frequency range. According to empirical predictions the largest kinetic energies are located around $ν_{\mathrm{max}} = 312.8\ μ\mathrm{Hz}$, but in both simulations the modes with frequencies $ν<50\ μ\mathrm{Hz}$ have the largest kinetic energies. In the simulation with $r/r_\star = 0.98$, the simulated modes have extrapolated surface velocities comparable to the empirical predictions, with highest surface velocities in a bell-shaped curve peaking around $ν= 700 \ μ\mathrm{Hz}$. The extrapolated surface velocities of the low frequency modes are small, and thus hard to observe, but their large kinetic energies deeper in the interior could significantly impact angular momentum transport, which has not yet been investigated. △ Less

Submitted 5 June, 2026; originally announced June 2026.

Comments: 18 pages, 7 figures, 1 table; accepted for publication in ApJ

Lithium-rich M-dwarfs at the ZAMS: Evidence for planetary engulfment?

Authors: R. D. Jeffries, R. J. Jackson, I. Baraffe

Abstract: We identify 6 early M-dwarfs, in 3 open clusters (NGC 2451a, Blanco 1 and NGC 2516) at ages of 50-200 Myr, that are anomalously enriched in lithium compared with Li-depleted siblings of similar spectral type. The Li-rich outliers represent 2-3 per cent of stars with $3560 < T_{\rm eff}/{\rm K} < 4045$ in clusters at those ages but are otherwise indistinguishable in their positions, parallaxes and… ▽ More We identify 6 early M-dwarfs, in 3 open clusters (NGC 2451a, Blanco 1 and NGC 2516) at ages of 50-200 Myr, that are anomalously enriched in lithium compared with Li-depleted siblings of similar spectral type. The Li-rich outliers represent 2-3 per cent of stars with $3560 < T_{\rm eff}/{\rm K} < 4045$ in clusters at those ages but are otherwise indistinguishable in their positions, parallaxes and kinematics from other cluster members; their placement in absolute colour-magnitude diagrams is incompatible with being much younger Li-rich interlopers, only one shows evidence of binarity and they are all slow rotators. The enhanced Li abundances are consistent with the engulfment of 3-10 $M_\oplus$ of volatile-depleted planetary material after the formation of a radiative core has ended rapid pre main sequence Li depletion. Published planetary formation simulations featuring engulfment via dynamical interactions, and the preponderance of Earth-like exoplanets in close orbits around M-dwarfs, offer some support to this scenario. The observed occurrence rate would be a lower limit to the frequency with which such engulfment events occur between ages of $\sim (30-200)$ Myr, that depends in the timescale for ongoing Li depletion at the ZAMS. △ Less

Submitted 25 May, 2026; originally announced May 2026.

Comments: Accepted version - for publication in MNRAS

Unveiling stellar (and planetary) internal dynamics with the fully compressible MUSIC code

Authors: Arthur Le Saux, Isabelle Baraffe, Thomas Guillet, Jane Pratt, Tom Goffrey, Dimitar Vlaykov, Adrien Morison, Jack Morton, Maxime Stuck, Mary Geer Dethero, Nils de Vries

Abstract: Multidimensional hydrodynamical simulations have transformed the study of stellar interiors over the past few decades. Most codes developed during that time use the anelastic approximation, which fixes the thermal structure of simulations and filters out sound waves. Many of them also use explicit time integration, which imposes severe constraints on the time step of the simulations. In this conte… ▽ More Multidimensional hydrodynamical simulations have transformed the study of stellar interiors over the past few decades. Most codes developed during that time use the anelastic approximation, which fixes the thermal structure of simulations and filters out sound waves. Many of them also use explicit time integration, which imposes severe constraints on the time step of the simulations. In this context, MUSIC is developed to overcome these limitations. Its main scientific objective is to improve the phenomenological approaches used in 1D stellar evolution codes to describe major hydrodynamical and MHD processes. Here, we review recent applications of the MUSIC code, that focus mainly on convection, convective boundary mixing and waves in stars that possess convective cores, shells and/or envelopes. △ Less

Submitted 27 October, 2025; originally announced October 2025.

Comments: Proceedings of the French Society for Astronomy and Astrophysics (SF2A) conference 2025

Mass dependence of overshooting beneath convective envelopes

Authors: J. Pratt, I. Baraffe, M. -G. Dethero, M. Stuck, D. G. Vlaykov, T. Goffrey

Abstract: Although the dependence of convective core overshooting on mass has attracted much attention, no corresponding work exists for overshooting below a convective envelope. We aim to quantify this relationship for pre-main sequence stars of intermediate mass ranging from $1.2 M_{\mathsf{sun}}$ to $6 M_{\mathsf{sun}}$. These stars have a similar thermal and density structure, making this a suitable cho… ▽ More Although the dependence of convective core overshooting on mass has attracted much attention, no corresponding work exists for overshooting below a convective envelope. We aim to quantify this relationship for pre-main sequence stars of intermediate mass ranging from $1.2 M_{\mathsf{sun}}$ to $6 M_{\mathsf{sun}}$. These stars have a similar thermal and density structure, making this a suitable choice to isolate the effect of changing mass. We produce a series of two-dimensional global simulations of stars using MUSIC, a fully compressible, time-implicit hydrodynamics code. The stars that we select for this study are near the end of the pre-main sequence and are convectively unstable above 80% of their stellar radius; they thus have a convective envelope that is shallower than the current sun. For this series of stellar models, a simple scaling with luminosity, with a scaling exponent of 1/4, accounts for the increasing overshooting with stellar mass. This result has interesting similarities with the scaling found by Baraffe et al. [2023] for a range of intermediate mass and massive stars at the zero-age main sequence (ZAMS) that have convective cores. △ Less

Submitted 22 September, 2025; originally announced September 2025.

Comments: 24 pages

Report number: LLNL-TR-2010799

Journal ref: Lawrence Livermore National Laboratory Technical Report FY-2025

Discovery of the pre-main-sequence eclipsing binary MML 48

Authors: Y. Gómez Maqueo Chew, L. Hebb, H. C. Stempels, F. M. Walter, D. J. James, G. A. Feiden, R. Petrucci, T. Lister, I. Baraffe, M. Brodheim, F. Faedi, D. R. Anderson, R. A. Street, C. Hellier, K. G. Stassun

Abstract: We present the discovery of the eclipsing binary MML 48, which is a member of Upper Centaurus Lupus, has an associated age of 16 Myr, and is composed of two young, low-mass stars. We used space- and ground-based observations to characterize the system with both time-series photometry and spectroscopy. Given the extreme mass ratio between the stars, q_EB = 0.209 +- 0.014, we modeled a single-lined… ▽ More We present the discovery of the eclipsing binary MML 48, which is a member of Upper Centaurus Lupus, has an associated age of 16 Myr, and is composed of two young, low-mass stars. We used space- and ground-based observations to characterize the system with both time-series photometry and spectroscopy. Given the extreme mass ratio between the stars, q_EB = 0.209 +- 0.014, we modeled a single-lined spectroscopic and eclipsing binary system. The orbital period, 2.0171068 +- 0.0000004 d, is measured from the highest precision light curves. We derive a primary mass of 1.2 +- 0.07 Msun using stellar models, and with radial velocities we measured a secondary mass of 0.2509 +- 0.0078 Msun. The radii are large, as expected for pre-main-sequence stars, and are measured as 1.574 +- 0.026 +- 0.050 Rsun and 0.587 +- 0.0095 +- 0.050 Rsun, for the primary and secondary stars, respectively. MML 48 joins the short list of known low-mass, pre-main-sequence eclipsing binaries (EBs), being one of only five systems with intermediate ages (15-25 Myr), and the system with the most extreme mass ratio. The primary star is currently at the "fusion bump", undergoing an over-production of energy in the core due to the build-up of 3He before reaching its equilibrium abundance set by the proton-proton (p-p) I chain. MML 48 A is the first young star in an eclipsing system that has been found during its fusion bump. MML 48 is thus an important benchmark for low-mass stellar evolution at a time when the stars are rapidly changing, which allows for a tight constraint on the corresponding isochrone given the uneven mass ratio. △ Less

Submitted 3 September, 2025; originally announced September 2025.

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

Extreme Lithium Depletion in Solar Twins: Challenging Non-Standard Mixing Models

Authors: Isabelle Winnick, Jhon Yana Galarza, Henrique Reggiani, Thiago Ferreira, Isabelle Baraffe, Diego Lorenzo-Oliveira, Micaela Oyague, Rita Valle, Renzo Trujillo Diaz, Nathan Leigh, Matias Flores Trivigno, Ricardo Lopez-Valdivia, Gabriela Carvalho Silva, Eder Martioli, Helio Perottoni

Abstract: Lithium (Li) is a powerful tracer of stellar mixing, gradually depleted in solar twins by non-standard transport below the convective zone. Here, we identify six new solar twins with exceptionally low Li levels that are not explained by current non-standard mixing models and, together with our previously reported anomalous solar twin HIP 8522, suggest a distinct population marked by a violent evol… ▽ More Lithium (Li) is a powerful tracer of stellar mixing, gradually depleted in solar twins by non-standard transport below the convective zone. Here, we identify six new solar twins with exceptionally low Li levels that are not explained by current non-standard mixing models and, together with our previously reported anomalous solar twin HIP 8522, suggest a distinct population marked by a violent evolutionary past. Employing high-resolution spectra ($R=60,000 - 165,000$), we infer precise stellar parameters and chemical compositions, including Li abundances. We consider possible scenarios generating enhanced mixing, including planetary engulfment, blue straggler stars (BSSs), and early episodic accretion. Our planet engulfment simulations indicate that only one star may have engulfed an exoplanet, rapidly depleting Li via thermohaline convection. In the BSS scenario, radial velocity data rule out binary mass transfer, revealing no stellar companions but instead two new exoplanets. If these stars are field BSSs, a binary merger is likely though uncertain given that current BSS models focus mostly on stars in open clusters. Using pre-main-sequence episodic accretion models, we find that solar-mass stars can experience enhanced Li depletion without significant beryllium (Be) depletion. This is consistent with the Be abundances measured in two of our stars and represents the most plausible scenario, pending Be measurements for the remaining stars. These unique stars, together with HIP 8522, represent exceptional cases for testing stellar evolution models and probing internal mixing processes in Sun-like stars. △ Less

Submitted 22 August, 2025; originally announced August 2025.

Comments: 32 pages, 12 figures, 9 tables, submitted to ApJ

Convective shells in the interior of Cepheid variable stars: overshooting models based on hydrodynamic simulations

Authors: Maxime Stuck, Jane Pratt, Isabelle Baraffe, Joyce Ann Guzik, Mary-Geer Dethero, Dimitar Vlaykov, Tom Goffrey, Arthur Le Saux

Abstract: Because Cepheid variable stars have long been used as a cosmic benchmark, the accuracy of stellar evolution models for Cepheids have wide-reaching effects. Our goal is to provide a detailed multi-dimensional picture of hydrodynamic convection and convective boundary mixing in the interior of Cepheids. We perform 2D hydrodynamic simulations of six stars with the fully compressible Multidimensional… ▽ More Because Cepheid variable stars have long been used as a cosmic benchmark, the accuracy of stellar evolution models for Cepheids have wide-reaching effects. Our goal is to provide a detailed multi-dimensional picture of hydrodynamic convection and convective boundary mixing in the interior of Cepheids. We perform 2D hydrodynamic simulations of six stars with the fully compressible Multidimensional Stellar Implicit Code (MUSIC). Our simulations do not model the radial pulsations but focus on the interior structure of Cepheids, which involves an interior convective shell and a convective envelope. We develop a new statistical analysis to examine overshooting in this inner convection zone. Using the extreme value theory, we find that overshooting above the convective shell fills the space between these convectively unstable layers. We develop a new statistical analysis that provides a clearer picture of how overshooting fills this layer, and also allows us to formulate a detailed comparison between overshooting above and below the convective shell. Our analysis effectively decomposes the overshooting layer into two layers: a weak and a strong overshooting layer. Statistically, this is accomplished by decomposing the strongly non-Gaussian probability density function into a mixture of Gamma distributions. Using our mixture model, we show that the ratio of overshooting lengths above and below the convective shell depends directly on the radial extent of the convective shell as well as its depth in the star. We propose a new form for the diffusion coefficient, which addresses the need for overlapping overshooting layers between convective shells. We introduce the idea of super-mixing layer where overshooting from both the convective shell and the convective envelope results in efficient mixing and could be viewed as merging the two adjacent convective zones. △ Less

Submitted 7 May, 2025; originally announced May 2025.

Comments: Accepted for publication in Astronomy & Astrophysics on May 6, 2025

Report number: LLNL-JRNL-2003963

Journal ref: A&A 698, A304 (2025)

Roadmap for warm dense matter physics

Authors: Jan Vorberger, Frank Graziani, David Riley, Andrew D. Baczewski, Isabelle Baraffe, Mandy Bethkenhagen, Simon Blouin, Maximilian P. Böhme, Michael Bonitz, Michael Bussmann, Alexis Casner, Witold Cayzac, Peter Celliers, Gilles Chabrier, Nicolas Chamel, Dave Chapman, Mohan Chen, Jean Clérouin, Gilbert Collins, Federica Coppari, Tilo Döppner, Tobias Dornheim, Luke B. Fletcher, Dirk O. Gericke, Siegfried Glenzer , et al. (49 additional authors not shown)

Abstract: This roadmap presents the state-of-the-art, current challenges and near future developments anticipated in the thriving field of warm dense matter physics. Originating from strongly coupled plasma physics, high pressure physics and high energy density science, the warm dense matter physics community has recently taken a giant leap forward. This is due to spectacular developments in laser technolog… ▽ More This roadmap presents the state-of-the-art, current challenges and near future developments anticipated in the thriving field of warm dense matter physics. Originating from strongly coupled plasma physics, high pressure physics and high energy density science, the warm dense matter physics community has recently taken a giant leap forward. This is due to spectacular developments in laser technology, diagnostic capabilities, and computer simulation techniques. Only in the last decade has it become possible to perform accurate enough simulations \& experiments to truly verify theoretical results as well as to reliably design experiments based on predictions. Consequently, this roadmap discusses recent developments and contemporary challenges that are faced by theoretical methods, and experimental techniques needed to create and diagnose warm dense matter. A large part of this roadmap is dedicated to specific warm dense matter systems and applications in astrophysics, inertial confinement fusion and novel material synthesis. △ Less

Submitted 13 February, 2026; v1 submitted 5 May, 2025; originally announced May 2025.

Mixing by Internal Gravity Waves in Stars: Assessing Numerical Simulations Against Theory

Authors: Jack Morton, Thomas Guillet, Isabelle Baraffe, Adrien Morison, Arthur Le Saux, Dimitar Vlaykov, Tom Goffrey, Jane Pratt

Abstract: Here we present a study of radial chemical mixing in non-rotating massive main-sequence stars driven by internal gravity waves (IGWs), based on multi-dimensional hydrodynamical simulations with the fully compressible code MUSIC. We examine two proposed mechanisms of material mixing in stars by IGWs that are commonly quoted, relating to thermal diffusion and sub-wavelength shearing. Thermal diffusi… ▽ More Here we present a study of radial chemical mixing in non-rotating massive main-sequence stars driven by internal gravity waves (IGWs), based on multi-dimensional hydrodynamical simulations with the fully compressible code MUSIC. We examine two proposed mechanisms of material mixing in stars by IGWs that are commonly quoted, relating to thermal diffusion and sub-wavelength shearing. Thermal diffusion provides a non-restorative effect to the waves, leaving material displaced from its previous equilibrium, while shearing arising within the waves drives weak localised flows, mixing the fluid there. Using IGW spectra from the simulations, we evaluate theoretical predictions of mixing rates due to these mechanisms. We show, for $20M_\odot$ main-sequence stars, that neither of these mechanisms are likely to create mixing sufficient to correct inaccuracies in current stellar evolution models. Furthermore, we compare these predictions to results obtained from Lagrangian tracer particles, following a method recently used for global simulations of stellar interiors to measure mixing by IGWs in their radiative zones. We demonstrate that tracer particle methods face significant numerical challenges in measuring the small diffusion coefficients predicted by the aforementioned theories, for which they are prone to yielding artificially enhanced coefficients. Diffusion coefficients based on such methods are currently used with stellar evolution codes for asteroseismic studies, but should be viewed with caution. Finally, in a case where tracer particles do not suffer from numerical artefacts, we suggest that a diffusion model is not suitable for timescales typically considered by two-dimensional numerical simulations. △ Less

Submitted 7 January, 2025; originally announced January 2025.

Comments: 18 pages, 13 figures; to be published in MNRAS

JWST/NIRSpec Observations of Brown Dwarfs in the Orion Nebula Cluster

Authors: K. L. Luhman, C Alves de Oliveira, I. Baraffe, G. Chabrier, E. Manjavacas, R. J. Parker, P. Tremblin

Abstract: We have used the multiobject mode of the Near-Infrared Spectrograph (NIRSpec) on board the James Webb Space Telescope (JWST) to obtain low-resolution 1-5um spectra of 22 brown dwarf candidates in the Orion Nebula Cluster, which were selected with archival images from the Hubble Space Telescope. One of the targets was previously classified as a Herbig-Haro (HH) object and exhibits strong emission i… ▽ More We have used the multiobject mode of the Near-Infrared Spectrograph (NIRSpec) on board the James Webb Space Telescope (JWST) to obtain low-resolution 1-5um spectra of 22 brown dwarf candidates in the Orion Nebula Cluster, which were selected with archival images from the Hubble Space Telescope. One of the targets was previously classified as a Herbig-Haro (HH) object and exhibits strong emission in H I, H2, and the fundamental band of CO, further demonstrating that HH objects can have bright emission in that CO band. The remaining targets have late spectral types (M6.5 to early L) and are young based on gravity sensitive features, as expected for low-mass members of the cluster. According to theoretical evolutionary models, these objects should have masses that range from the hydrogen burning limit to 0.003-0.007 Msun. Two of the NIRSpec targets were identified as proplyds in earlier analysis of Hubble images. They have spectral types of M6.5 and M7.5, making them two of the coolest and least massive known proplyds. Another brown dwarf shows absorption bands at 3-5um from ices containing H2O, CO2, OCN-, and CO, indicating that it is either an edge-on class II system or a class I protostar. It is the coolest and least massive object that has detections of these ice features. In addition, it appears to be the first candidate for a protostellar brown dwarf that has spectroscopy confirming its late spectral type. △ Less

Submitted 13 October, 2024; originally announced October 2024.

Comments: Astronomical Journal, in press

The shape of convection in 2D and 3D global simulations of stellar interiors

Authors: M. -G. Dethero, J. Pratt, D. G. Vlaykov, I. Baraffe, T. Guillet, T. Goffrey, A. Le Saux, A. Morison

Abstract: Theoretical descriptions of convective overshooting often rely on a one-dimensional parameterization of the flow called the filling factor for convection. Several definitions of the filling factor have been developed, based on: (1) the percentage of the volume, (2) the mass flux, and (3) the convective flux that moves through the boundary. We examine these definitions of the filling factor with th… ▽ More Theoretical descriptions of convective overshooting often rely on a one-dimensional parameterization of the flow called the filling factor for convection. Several definitions of the filling factor have been developed, based on: (1) the percentage of the volume, (2) the mass flux, and (3) the convective flux that moves through the boundary. We examine these definitions of the filling factor with the goal of establishing their ability to explain differences between 2D and 3D global simulations of stellar interiors that include fully compressible hydrodynamics and realistic microphysics for stars. We study pairs of identical two- and three-dimensional global simulations of stars produced with MUSIC, a fully compressible, time-implicit hydrodynamics code. We examine (1) a $3 M_\odot$ red giant star near the first dredge-up point, (2) a $1 M_\odot$ pre-main-sequence star with a large convection zone, (3) the current sun, and (4) a $20 M_\odot$ main-sequence star with a large convective core. Our calculations of the filling factor based on the volume percentage and the mass flux indicate asymmetrical convection near the surface for each star with an outer convection zone. However, near the convective boundary, convective flows achieve inward-outward symmetry; for 2D and 3D simulations, these filling factors are indistinguishable. A filling factor based on the convective flux is contaminated by boundary-layer-like flows, making theoretical interpretation difficult. We present two new alternatives to these standard definitions, which compare flows at two different radial points. The first is the penetration parameter of Anders et al. (2022). The second is a new statistic, the plume interaction parameter. We demonstrate that both of these parameters capture systematic differences between 2D and 3D simulations around the convective boundary. △ Less

Submitted 15 September, 2024; originally announced September 2024.

Comments: 15 pages, 16 figures

Report number: LLNL-JRNL-857704

Journal ref: A&A 692, A46 (2024)

Effects of stratification on overshooting and waves atop the convective core of $5M_{\odot}$ main-sequence stars

Authors: Adrien Morison, Arthur Le Saux, Isabelle Baraffe, Jack Morton, Thomas Guillet, Dimitar Vlaykov, Tom Goffrey, Jane Pratt

Abstract: As a massive star evolves along the main sequence, its core contracts, leaving behind a stable stratification in helium. We simulate 2D convection in the core at three different stages of evolution of a $5M_{\odot}$ star, with three different stratifications in helium atop the core. We study the propagation of internal gravity waves in the stably-stratified envelope, along with the overshooting le… ▽ More As a massive star evolves along the main sequence, its core contracts, leaving behind a stable stratification in helium. We simulate 2D convection in the core at three different stages of evolution of a $5M_{\odot}$ star, with three different stratifications in helium atop the core. We study the propagation of internal gravity waves in the stably-stratified envelope, along with the overshooting length of convective plumes above the convective boundary. We find that the stratification in helium in evolved stars hinders radial motions and effectively shields the radiative envelope against plume penetration. This prevents convective overshooting from being an efficient mixing process in the radiative envelope. In addition, internal gravity waves are less excited in evolved models compared to the zero-age-main-sequence model, and are also more damped in the stratified region above the core. As a result, the wave power is several orders of magnitude lower in mid- and terminal-main-sequence models compared to zero-age-main-sequence stars. △ Less

Submitted 8 July, 2024; originally announced July 2024.

JWST/NIRSpec Observations of the Coldest Known Brown Dwarf

Authors: K. L. Luhman, P. Tremblin, C. Alves de Oliveira, S. M. Birkmann, I. Baraffe, G. Chabrier, E. Manjavacas, R. J. Parker, J. Valenti

Abstract: We present 1-5um spectroscopy of the coldest known brown dwarf, WISE J085510.83-071442.5 (WISE 0855), performed with the Near-Infrared Spectrograph (NIRSpec) on board the James Webb Space Telescope (JWST). NIRSpec has dramatically improved the measurement of spectral energy distribution of WISE 0855 in terms of wavelength coverage, signal-to-noise ratios, and spectral resolution. We have performed… ▽ More We present 1-5um spectroscopy of the coldest known brown dwarf, WISE J085510.83-071442.5 (WISE 0855), performed with the Near-Infrared Spectrograph (NIRSpec) on board the James Webb Space Telescope (JWST). NIRSpec has dramatically improved the measurement of spectral energy distribution of WISE 0855 in terms of wavelength coverage, signal-to-noise ratios, and spectral resolution. We have performed preliminary modeling of the NIRSpec data using the ATMO 2020 models of cloudless atmospheres, arriving at a best fitting model that has T_eff=285 K. That temperature is ~20 K higher than the value derived by combining our luminosity estimate with evolutionary models (i.e., the radius in the model fit to the SED is somewhat smaller than expected from evolutionary models). Through comparisons to the model spectra, we detect absorption in the fundamental band of CO, which is consistent with an earlier detection in a ground-based spectrum and indicates the presence of vertical mixing. Although PH_3 is expected in Y dwarfs that experience vertical mixing, it is not detected in WISE 0855. Previous ground-based M-band spectroscopy of WISE 0855 has been cited for evidence of H_2O ice clouds, but we find that the NIRSpec data in that wavelength range are matched well by our cloudless model. Thus, clear evidence of H_2O ice clouds in WISE 0855 has not been identified yet, but it may still be present in the NIRSpec data. The physical properties of WISE 0855, including the presence of H_2O clouds, can be better constrained by more detailed fitting with both cloudless and cloudy models and the incorporation of unpublished 5-28um data from the Mid-infrared Instrument on JWST. △ Less

Submitted 28 November, 2023; originally announced November 2023.

Comments: Astronomical Journal, in press

First light of VLT/HiRISE: High-resolution spectroscopy of young giant exoplanets

Authors: A. Vigan, M. El Morsy, M. Lopez, G. P. P. L. Otten, J. Garcia, J. Costes, E. Muslimov, A. Viret, Y. Charles, G. Zins, G. Murray, A. Costille, J. Paufique, U. Seemann, M. Houllé, H. Anwand-Heerwart, M. Phillips, A. Abinanti, P. Balard, I. Baraffe, J. -A. Benedetti, P. Blanchard, L. Blanco, J. -L. Beuzit, E. Choquet , et al. (24 additional authors not shown)

Abstract: A major endeavor of this decade is the direct characterization of young giant exoplanets at high spectral resolution to determine the composition of their atmosphere and infer their formation processes and evolution. Such a goal represents a major challenge owing to their small angular separation and luminosity contrast with respect to their parent stars. Instead of designing and implementing comp… ▽ More A major endeavor of this decade is the direct characterization of young giant exoplanets at high spectral resolution to determine the composition of their atmosphere and infer their formation processes and evolution. Such a goal represents a major challenge owing to their small angular separation and luminosity contrast with respect to their parent stars. Instead of designing and implementing completely new facilities, it has been proposed to leverage the capabilities of existing instruments that offer either high contrast imaging or high dispersion spectroscopy, by coupling them using optical fibers. In this work we present the implementation and first on-sky results of the HiRISE instrument at the very large telescope (VLT), which combines the exoplanet imager SPHERE with the recently upgraded high resolution spectrograph CRIRES using single-mode fibers. The goal of HiRISE is to enable the characterization of known companions in the $H$ band, at a spectral resolution of the order of $R = λ/Δλ= 100\,000$, in a few hours of observing time. We present the main design choices and the technical implementation of the system, which is constituted of three major parts: the fiber injection module inside of SPHERE, the fiber bundle around the telescope, and the fiber extraction module at the entrance of CRIRES. We also detail the specific calibrations required for HiRISE and the operations of the instrument for science observations. Finally, we detail the performance of the system in terms of astrometry, temporal stability, optical aberrations, and transmission, for which we report a peak value of $\sim$3.9% based on sky measurements in median observing conditions. Finally, we report on the first astrophysical detection of HiRISE to illustrate its potential. △ Less

Submitted 22 November, 2023; v1 submitted 21 September, 2023; originally announced September 2023.

Comments: 17 pages, 15 figures, 3 tables. Accepted in A&A on 24 October 2023

Evidence of Radius Inflation in Radiative GCM Models of WASP-76b due to the Advection of Potential Temperature

Authors: Felix Sainsbury-Martinez, Pascal Tremblin, Aaron David Schneider, Ludmila Carone, Isabelle Baraffe, Gilles Chabrier, Christiane Helling, Leen Decin, Uffe Gråe Jørgensen

Abstract: Understanding the discrepancy between the radii of observed hot Jupiters and standard 'radiative-convective' models remains a hotly debated topic in the exoplanet community. One mechanism which has been proposed to bridge this gap, and which has recently come under scrutiny, is the vertical advection of potential temperature from the irradiated outer atmosphere deep into the interior, heating the… ▽ More Understanding the discrepancy between the radii of observed hot Jupiters and standard 'radiative-convective' models remains a hotly debated topic in the exoplanet community. One mechanism which has been proposed to bridge this gap, and which has recently come under scrutiny, is the vertical advection of potential temperature from the irradiated outer atmosphere deep into the interior, heating the deep, unirradiated, atmosphere, warming the internal adiabat, and resulting in radius inflation. Specifically, a recent study which explored the atmosphere of WASP-76b using a 3D, non-grey, GCM suggested that their models lacked radius inflation, and hence any vertical enthalpy advection. Here we perform additional analysis of these, and related, models, focusing on an explicit analysis of vertical enthalpy transport and the resulting heating of the deep atmosphere compared with 1D models. Our results indicate that, after any evolution linked with initialisation, all the WASP-76b models considered here exhibit significant vertical enthalpy transport, heating the deep atmosphere significantly when compared with standard 1D models. Furthermore, comparison of a long time-scale (and hence near steady-state) model with a Jupiter-like internal-structure model suggests not only strong radius-inflation, but also that the model radius, $1.98 \mathrm{R_{J}}$, may be comparable with observations ($1.83\pm0.06 \mathrm{R_{J}}$). We thus conclude that the vertical advection of potential temperature alone is enough to explain the radius inflation of WASP-76b, and potentially other irradiated gas giants, albeit with the proviso that the exact strength of the vertical advection remains sensitive to model parameters, such as the inclusion of deep atmospheric drag. △ Less

Submitted 21 June, 2023; originally announced June 2023.

Comments: Accepted for publication in MNRAS

Two-dimensional simulations of internal gravity waves in a 5 $M_{\odot}$ Zero-Age-Main-Sequence model

Authors: A. Le Saux, I. Baraffe, T. Guillet, D. G. Vlaykov, A. Morison, J. Pratt, T. Constantino, T. Goffrey

Abstract: Main-sequence intermediate-mass stars present a radiative envelope that supports internal gravity waves (IGWs). Excited at the boundary with the convective core, IGWs propagate towards the stellar surface and are suspected to impact physical processes such as rotation and chemical mixing. Using the fully compressible time-implicit code MUSIC, we study IGWs in two-dimensional simulations of a zero-… ▽ More Main-sequence intermediate-mass stars present a radiative envelope that supports internal gravity waves (IGWs). Excited at the boundary with the convective core, IGWs propagate towards the stellar surface and are suspected to impact physical processes such as rotation and chemical mixing. Using the fully compressible time-implicit code MUSIC, we study IGWs in two-dimensional simulations of a zero-age-main-sequence 5 solar mass star model up to 91\% of the stellar radius with different luminosity and radiative diffusivity enhancements. Our results show that low frequency waves excited by core convection are strongly impacted by radiative effects as they propagate. This impact depends on the radial profile of radiative diffusivity which increases by almost 5 orders of magnitude between the centre of the star and the top of the simulation domain. In the upper layers of the simulation domain, we observe an increase of the temperature. Our study suggests that this is due to heat added in these layers by IGWs damped by radiative diffusion. We show that non-linear effects linked to large amplitude IGWs may be relevant just above the convective core. Both these effects are intensified by the artificial enhancement of the luminosity and radiative diffusivity, with enhancement factors up to $10^4$ times the realistic values. Our results also highlight that direct comparison between numerical simulations with enhanced luminosity and observations must be made with caution. Finally, our work suggests that thermal effects linked to the damping of IGWs could have a non-negligible impact on stellar structure. △ Less

Submitted 5 April, 2023; originally announced April 2023.

Comments: 15 pages, 10 figures, accepted for publication in MNRAS

A study of convective core overshooting as a function of stellar mass based on two-dimensional hydrodynamical simulations

Authors: I. Baraffe, J. Clarke, A. Morison, D. G. Vlaykov, T. Constantino, T. Goffrey, T. Guillet, A. Le Saux, J. Pratt

Abstract: We perform two-dimensional numerical simulations of core convection for zero-age-main-sequence stars covering a mass range from 3 $M_\odot$ to 20 $M_\odot$. The simulations are performed with the fully compressible time-implicit code MUSIC. We study the efficiency of overshooting, which describes the ballistic process of convective flows crossing a convective boundary, as a function of stellar mas… ▽ More We perform two-dimensional numerical simulations of core convection for zero-age-main-sequence stars covering a mass range from 3 $M_\odot$ to 20 $M_\odot$. The simulations are performed with the fully compressible time-implicit code MUSIC. We study the efficiency of overshooting, which describes the ballistic process of convective flows crossing a convective boundary, as a function of stellar mass and luminosity. We also study the impact of artificially increasing the stellar luminosity for 3 $M_\odot$ models. The simulations cover hundreds to thousands of convective turnover timescales. Applying the framework of extreme plume events previously developed for convective envelopes, we derive overshooting lengths as a function of stellar masses. We find that the overshooting distance ($d_{\rm ov}$) scales with the stellar luminosity ($L$) and the convective core radius ($r_{\rm conv}$). We derive a scaling law $d_{\rm ov} \propto L^{1/3} r_{\rm conv}^{1/2}$ which is implemented in a 1D stellar evolution code and the resulting stellar models are compared to observations. The scaling predicts values for the overshooting distance that significantly increase with stellar mass, in qualitative agreement with observations. Quantitatively, however, the predicted values are underestimated for masses $\gtrsim 10 M_\odot$. Our 2D simulations show the formation of a nearly-adiabatic layer just above the Schwarzschild boundary of the convective core, as exhibited in recent 3D simulations of convection. The most luminous models show a growth in size with time of the nearly-adiabatic layer. This growth seems to slow down as the upper edge of the nearly-adiabatic layer gets closer to the maximum overshooting length and as the simulation time exceeds the typical thermal diffusive timescale in the overshooting layer. △ Less

Submitted 6 January, 2023; originally announced January 2023.

Comments: 13 pages, accepted for publication in MNRAS

Impact of a new H/He equation of state on the evolution of massive brown dwarfs. New determination of the hydrogen burning limit

Authors: G. Chabrier, I. Baraffe, M. Phillips, F. Debras

Abstract: We have explored the impact of the latest equation of state (EOS) for dense hydrogen-helium mixtures (Chabrier \& Debras 2021), which takes into account the interactions between hydrogen and helium species, upon the evolution of very low mass stars and brown dwarfs (BD). These interactions modify the thermodynamic properties of the H/He mixture, notably the entropy, a quantity of prime importance… ▽ More We have explored the impact of the latest equation of state (EOS) for dense hydrogen-helium mixtures (Chabrier \& Debras 2021), which takes into account the interactions between hydrogen and helium species, upon the evolution of very low mass stars and brown dwarfs (BD). These interactions modify the thermodynamic properties of the H/He mixture, notably the entropy, a quantity of prime importance for these fully convective bodies, but also the onset and the development of degeneracy throughout the body. This translates into a faster cooling rate, i.e. cooler isentropes for a given mass and age, and thus larger brown dwarf masses and smaller radii for given effective temperature and luminosity than the models based on previous EOSs. This means that objects of a given mass and age, in the range $M\lesssim 0.1\,\msol$, $τ\gtrsim 10^8$ yr, will have cooler effective temperatures and fainter luminosities. Confronting these new models with several observationally determined BD dynamical masses, we show that this improves the agreement between evolutionary models and observations and resolves at least part of the observed discrepancies between the properties of dynamical mass determinations and evolutionary models. A noticeable consequence of this improvement of the dense H/He EOS is that it yields a larger H-burning minimum mass, now found to be $0.075\,\msol$ ($78.5\,\mjup$) with the ATMO atmosphere models for solar metallicity. These updated brown dwarf models are made publicly available. △ Less

Submitted 14 December, 2022; originally announced December 2022.

Comments: To appear in Astronomy & Astrophysics

Connecting SPHERE and CRIRES+ for the characterisation of young exoplanets at high spectral resolution: status update of VLT/HiRISE

Authors: A. Vigan, M. Lopez, M. El Morsy, E. Muslimov, A. Viret, G. Zins, G. Murray, A. Costille, G. P. P. L. Otten, U. Seemann, H. Anwand-Heerwart, K. Dohlen, P. Blanchard, J. Garcia, Y. Charles, N. Tchoubaklian, T. Ely, M. Phillips, J. Paufique, J. -L. Beuzit, M. Houllé, J. Costes, R. Pourcelot, I. Baraffe, R. Dorn , et al. (10 additional authors not shown)

Abstract: New generation exoplanet imagers on large ground-based telescopes are highly optimised for the detection of young giant exoplanets in the near-infrared, but they are intrinsically limited for their characterisation by the low spectral resolution of their integral field spectrographs ($R<100$). High-dispersion spectroscopy at $R \gg 10^4$ would be a powerful tool for the characterisation of these p… ▽ More New generation exoplanet imagers on large ground-based telescopes are highly optimised for the detection of young giant exoplanets in the near-infrared, but they are intrinsically limited for their characterisation by the low spectral resolution of their integral field spectrographs ($R<100$). High-dispersion spectroscopy at $R \gg 10^4$ would be a powerful tool for the characterisation of these planets, but there is currently no high-resolution spectrograph with extreme adaptive optics and coronagraphy that would enable such characterisation. With project HiRISE we propose to use fiber coupling to combine the capabilities of two flagship instruments at the Very Large Telescope in Chile: the exoplanet imager SPHERE and the high-resolution spectrograph CRIRES+. The coupling will be implemented at the telescope in early 2023. We provide a general overview of the implementation of HiRISE, of its assembly, integration and testing (AIT) phase in Europe, and a brief assessment of its expected performance based on the final hardware. △ Less

Submitted 13 July, 2022; originally announced July 2022.

Comments: Submitted to SPIE Astronomical Telescopes + Instrumentation 2022, Adaptive Optics Systems VIII, Paper 12185-27

The JWST Early Release Science Program for the Direct Imaging & Spectroscopy of Exoplanetary Systems

Authors: Sasha Hinkley, Aarynn L. Carter, Shrishmoy Ray, Andrew Skemer, Beth Biller, Elodie Choquet, Maxwell A. Millar-Blanchaer, Stephanie Sallum, Brittany Miles, Niall Whiteford, Polychronis Patapis, Marshall D. Perrin, Laurent Pueyo, Glenn Schneider, Karl Stapelfeldt, Jason Wang, Kimberly Ward-Duong, Brendan P. Bowler, Anthony Boccaletti, Julien H. Girard, Dean Hines, Paul Kalas, Jens Kammerer, Pierre Kervella, Jarron Leisenring , et al. (61 additional authors not shown)

Abstract: The direct characterization of exoplanetary systems with high contrast imaging is among the highest priorities for the broader exoplanet community. As large space missions will be necessary for detecting and characterizing exo-Earth twins, developing the techniques and technology for direct imaging of exoplanets is a driving focus for the community. For the first time, JWST will directly observe e… ▽ More The direct characterization of exoplanetary systems with high contrast imaging is among the highest priorities for the broader exoplanet community. As large space missions will be necessary for detecting and characterizing exo-Earth twins, developing the techniques and technology for direct imaging of exoplanets is a driving focus for the community. For the first time, JWST will directly observe extrasolar planets at mid-infrared wavelengths beyond 5$μ$m, deliver detailed spectroscopy revealing much more precise chemical abundances and atmospheric conditions, and provide sensitivity to analogs of our solar system ice-giant planets at wide orbital separations, an entirely new class of exoplanet. However, in order to maximise the scientific output over the lifetime of the mission, an exquisite understanding of the instrumental performance of JWST is needed as early in the mission as possible. In this paper, we describe our 55-hour Early Release Science Program that will utilize all four JWST instruments to extend the characterisation of planetary mass companions to $\sim$15$μ$m as well as image a circumstellar disk in the mid-infrared with unprecedented sensitivity. Our program will also assess the performance of the observatory in the key modes expected to be commonly used for exoplanet direct imaging and spectroscopy, optimize data calibration and processing, and generate representative datasets that will enable a broad user base to effectively plan for general observing programs in future cycles. △ Less

Submitted 12 September, 2022; v1 submitted 25 May, 2022; originally announced May 2022.

Comments: 21 pages, 7 figures. Accepted for Publication in PASP

Impact of radial truncation on global 2D hydrodynamic simulations for a Sun-like model

Authors: D. G. Vlaykov, I. Baraffe, T. Constantino, T. Goffrey, T. Guillet, A. Le Saux, A. Morison, J. Pratt

Abstract: Stellar convection is a non-local process responsible for the transport of heat and chemical species. It can lead to enhanced mixing through convective overshooting and excitation of internal gravity waves (IGWs) at convective boundaries. The relationship between these processes is still not well understood and requires global hydrodynamic simulations to capture the important large-scale dynamics.… ▽ More Stellar convection is a non-local process responsible for the transport of heat and chemical species. It can lead to enhanced mixing through convective overshooting and excitation of internal gravity waves (IGWs) at convective boundaries. The relationship between these processes is still not well understood and requires global hydrodynamic simulations to capture the important large-scale dynamics. The steep stratification in stellar interiors suggests that the radial extent of such simulations can affect the convection dynamics, the IGWs in the stably stratified radiative zone, and the depth of the overshooting layer. We investigate these effects using two-dimensional global simulations performed with the fully compressible stellar hydrodynamics code MUSIC. We compare eight different radial truncations of the same solar-like stellar model evolved over approximately 400 convective turnover times. We find that the location of the inner boundary has an insignificant effect on the convection dynamics, the convective overshooting and the travelling IGWs. We relate this to the background conditions at the lower convective boundary which are unaffected by the truncation, as long as a significantly deep radiative layer is included in the simulation domain. However, we find that extending the outer boundary by only a few percent of the stellar radius significantly increases the velocity and temperature perturbations in the convection zone, the overshooting depth, the power and the spectral slope of the IGWs. The effect is related to the background conditions at the outer boundary, which are determined in essence by the hydrostatic stratification and the given luminosity. △ Less

Submitted 9 May, 2022; originally announced May 2022.

Comments: Accepted in MNRAS 2022 April 29

Two-dimensional simulations of solar-like models with artificially enhanced luminosity. II. Impact on internal gravity waves

Authors: A. Le Saux, T. Guillet, I. Baraffe, D. G. Vlaykov, T. Constantino, J. Pratt, T. Goffrey, M. Sylvain, V. Réville, A. S. Brun

Abstract: Artificially increasing the luminosity and the thermal diffusivity of a model is a common tactic adopted in hydrodynamical simulations of stellar convection. In this work, we analyse the impact of these artificial modifications on the physical properties of stellar interiors and specifically on internal gravity waves. We perform two-dimensional simulations of solar-like stars with the MUSIC code.… ▽ More Artificially increasing the luminosity and the thermal diffusivity of a model is a common tactic adopted in hydrodynamical simulations of stellar convection. In this work, we analyse the impact of these artificial modifications on the physical properties of stellar interiors and specifically on internal gravity waves. We perform two-dimensional simulations of solar-like stars with the MUSIC code. We compare three models with different luminosity enhancement factors to a reference model. The results confirm that properties of the waves are impacted by the artificial enhancement of the luminosity and thermal diffusivity. We find that an increase in the stellar luminosity yields a decrease in the bulk convective turnover timescale and an increase in the characteristic frequency of excitation of the internal waves. We also show that a higher energy input in a model, corresponding to a larger luminosity, results in higher energy in high frequency waves. Across our tests with the luminosity and thermal diffusivity enhanced together by up to a factor of 104, our results are consistent with theoretical predictions of radiative damping. Increasing the luminosity also has an impact on the amplitude of oscillatory motions across the convective boundary. One must use caution when interpreting studies of internal gravity waves based on hydrodynamical simulations with artificially enhanced luminosity. △ Less

Submitted 1 February, 2022; originally announced February 2022.

Comments: 16 pages, 13 figures; accepted for publication in A&A

Journal ref: A&A 660, A51 (2022)

Dynamics in a stellar convective layer and at its boundary: Comparison of five 3D hydrodynamics codes

Authors: R. Andrassy, J. Higl, H. Mao, M. Mocák, D. G. Vlaykov, W. D. Arnett, I. Baraffe, S. W. Campbell, T. Constantino, P. V. F. Edelmann, T. Goffrey, T. Guillet, F. Herwig, R. Hirschi, L. Horst, G. Leidi, C. Meakin, J. Pratt, F. Rizzuti, F. K. Roepke, P. Woodward

Abstract: Our ability to predict the structure and evolution of stars is in part limited by complex, 3D hydrodynamic processes such as convective boundary mixing. Hydrodynamic simulations help us understand the dynamics of stellar convection and convective boundaries. However, the codes used to compute such simulations are usually tested on extremely simple problems and the reliability and reproducibility o… ▽ More Our ability to predict the structure and evolution of stars is in part limited by complex, 3D hydrodynamic processes such as convective boundary mixing. Hydrodynamic simulations help us understand the dynamics of stellar convection and convective boundaries. However, the codes used to compute such simulations are usually tested on extremely simple problems and the reliability and reproducibility of their predictions for turbulent flows is unclear. We define a test problem involving turbulent convection in a plane-parallel box, which leads to mass entrainment from, and internal-wave generation in, a stably stratified layer. We compare the outputs from the codes FLASH, MUSIC, PPMSTAR, PROMPI, and SLH, which have been widely employed to study hydrodynamic problems in stellar interiors. The convection is dominated by the largest scales that fit into the simulation box. All time-averaged profiles of velocity components, fluctuation amplitudes, and fluxes of enthalpy and kinetic energy are within $\lesssim 3σ$ of the mean of all simulations on a given grid ($128^3$ and $256^3$ grid cells), where $σ$ describes the statistical variation due to the flow's time dependence. They also agree well with a $512^3$ reference run. The $128^3$ and $256^3$ simulations agree within $9\%$ and $4\%$, respectively, on the total mass entrained into the convective layer. The entrainment rate appears to be set by the amount of energy that can be converted to work in our setup and details of the small-scale flows in the boundary layer seem to be largely irrelevant. Our results lend credence to hydrodynamic simulations of flows in stellar interiors. We provide in electronic form all outputs of our simulations as well as all information needed to reproduce or extend our study. △ Less

Submitted 26 January, 2022; v1 submitted 1 November, 2021; originally announced November 2021.

Comments: 20 pages, 19 figures, new Fig. B.2, new clarifications, electronic materials updated, accepted for publication in A&A

Journal ref: A&A 659, A193 (2022)

Two-dimensional simulations of solar-like models with artificially enhanced luminosity -- I. Impact on convective penetration

Authors: I. Baraffe, J. Pratt, D. G. Vlaykov, T. Guillet, T. Goffrey, A. Le Saux, T. Constantino

Abstract: We perform 2D, fully compressible, time-implicit simulations of convection in a solar-like model with the MUSIC code. Our main motivation is to explore the impact of a common tactic adopted in numerical simulations of convection that use realistic stellar conditions. This tactic is to artificially increase the luminosity and to modify the thermal diffusivity of the reference stellar model. This wo… ▽ More We perform 2D, fully compressible, time-implicit simulations of convection in a solar-like model with the MUSIC code. Our main motivation is to explore the impact of a common tactic adopted in numerical simulations of convection that use realistic stellar conditions. This tactic is to artificially increase the luminosity and to modify the thermal diffusivity of the reference stellar model. This work focuses on the impact of these modifications on convective penetration (or overshooting) at the base of the convective envelope of a solar-like model. We explore a range of enhancement factors for the energy input and confirm the increase in the characteristic overshooting depth with the increase in the energy input. Our results highlight the importance of the impact of penetrative downflows on the thermal background below the convective boundary. This is a result of compression and shear which induce local heating and thermal mixing. The artificial increase in the energy flux intensifies the heating process by increasing the velocities in the convective zone and at the convective boundary, revealing a subtle connection between the local heating of the thermal background and the plume dynamics. This heating also increases the efficiency of heat transport by radiation which may counterbalance further heating and helps to establish a steady state. The modification of the thermal background by penetrative plumes impacts the width of the overshooting layer. Our results suggest that an artificial modification of the radiative diffusivity in the overshooting layer, rather than only accelerating the thermal relaxation, could also alter the dynamics of the penetrating plumes and thus the width of the overshooting layer. Results from simulations with an artificial modification of the energy flux and of the thermal diffusivity should be regarded with caution if used to determine an overshooting width. △ Less

Submitted 2 September, 2021; originally announced September 2021.

Comments: 13 pages, 14 figures; accepted for publication in A&A

Journal ref: A&A 654, A126 (2021)

Suppression of lithium depletion in young low-mass stars from fast rotation

Authors: Thomas Constantino, Isabelle Baraffe, Thomas Goffrey, Jane Pratt, Thomas Guillet, Dimitar Vlaykov, Louis Amard

Abstract: We compute rotating 1D stellar evolution models that include a modified temperature gradient in convection zones and criterion for convective instability inspired by rotating 3D hydrodynamical simulations performed with the MUSIC code. In those 3D simulations we found that convective properties strongly depend on the Solberg-Høiland criterion for stability. We therefore incorporated this into 1D s… ▽ More We compute rotating 1D stellar evolution models that include a modified temperature gradient in convection zones and criterion for convective instability inspired by rotating 3D hydrodynamical simulations performed with the MUSIC code. In those 3D simulations we found that convective properties strongly depend on the Solberg-Høiland criterion for stability. We therefore incorporated this into 1D stellar evolution models by replacing the usual Schwarzschild criterion for stability and also modifying the temperature gradient in convection zones. We computed a grid of 1D models between 0.55 and 1.2 stellar masses from the pre-main sequence to the end of main sequence in order to study the problem of lithium depletion in low-mass main sequence stars. This is an ideal test case because many of those stars are born as fast rotators and the rate of lithium depletion is very sensitive to the changes in the stellar structure. Additionally, observations show a correlation between slow rotation and lithium depletion, contrary to expectations from standard models of rotationally driven mixing. By suppressing convection, and therefore decreasing the temperature at the base of the convective envelope, lithium burning is strongly quenched in our rapidly rotating models to an extent sufficient to account for the lithium spread observed in young open clusters. △ Less

Submitted 23 August, 2021; originally announced August 2021.

Comments: 11 pages, 5 figures; accepted for publication in Astronomy & Astrophysics

Journal ref: A&A 654, A146 (2021)

Direct imaging and spectroscopy of exoplanets with the ELT/HARMONI high-contrast module

Authors: Mathis Houllé, Arthur Vigan, Alexis Carlotti, Élodie Choquet, Faustine Cantalloube, Mark W. Phillips, Jean-François Sauvage, Noah Schwartz, Gilles P. P. L. Otten, Isabelle Baraffe, Alexandre Emsenhuber, Christoph Mordasini

Abstract: Combining high-contrast imaging with medium-resolution spectroscopy has been shown to significantly boost the direct detection of exoplanets. HARMONI, one of the first-light instruments to be mounted on ESO's ELT, will be equipped with a single-conjugated adaptive optics system to reach the diffraction limit of the ELT in H and K bands, a high-contrast module dedicated to exoplanet imaging, and a… ▽ More Combining high-contrast imaging with medium-resolution spectroscopy has been shown to significantly boost the direct detection of exoplanets. HARMONI, one of the first-light instruments to be mounted on ESO's ELT, will be equipped with a single-conjugated adaptive optics system to reach the diffraction limit of the ELT in H and K bands, a high-contrast module dedicated to exoplanet imaging, and a medium-resolution (up to R = 17 000) optical and near-infrared integral field spectrograph. Combined together, these systems will provide unprecedented contrast limits at separations between 50 and 400 mas. In this paper, we estimate the capabilities of the HARMONI high-contrast module for the direct detection of young giant exoplanets. We use an end-to-end model of the instrument to simulate observations based on realistic observing scenarios and conditions. We analyze these data with the so-called "molecule mapping" technique combined to a matched-filter approach, in order to disentangle the companions from the host star and tellurics, and increase the S/N of the planetary signal. We detect planets above 5-sigma at contrasts up to 16 mag and separations down to 75 mas in several spectral configurations of the instrument. We show that molecule mapping allows the detection of companions up to 2.5 mag fainter compared to state-of-the-art high-contrast imaging techniques based on angular differential imaging. We also demonstrate that the performance is not strongly affected by the spectral type of the host star, and that we reach close sensitivities for the best three quartiles of observing conditions at Armazones, which means that HARMONI could be used in near-critical observations during 60 to 70% of telescope time at the ELT. Finally, we simulate planets from population synthesis models to further explore the parameter space that HARMONI and its high-contrast module will soon open. △ Less

Submitted 22 April, 2021; originally announced April 2021.

Comments: Accepted for publication in A&A (14 pages, 9 figures)

Journal ref: A&A 652, A67 (2021)

Rotational spectral modulation of cloudless atmospheres for L/T Brown Dwarfs and Extrasolar Giant Planets

Authors: P. Tremblin, M. W. Phillips, A. Emery, I. Baraffe, B. W. P. Lew, D. Apai, B. A. Biller, M. Bonnefoy

Abstract: The rotational spectral modulation (spectro-photometric variability) of brown dwarfs is usually interpreted as a sign of the presence of inhomogeneous cloud covers in the atmosphere. This paper aims at exploring the role of temperature fluctuations in these spectral modulations. These fluctuations could naturally arise in a convective atmosphere impacted by diabatic processes such as complex chemi… ▽ More The rotational spectral modulation (spectro-photometric variability) of brown dwarfs is usually interpreted as a sign of the presence of inhomogeneous cloud covers in the atmosphere. This paper aims at exploring the role of temperature fluctuations in these spectral modulations. These fluctuations could naturally arise in a convective atmosphere impacted by diabatic processes such as complex chemistry, i.e. the recently proposed mechanism to explain the L/T transition: CO/CH4 radiative convection. We use the 1D radiative/convective code ATMO with ad-hoc modifications of the temperature gradient to model the rotational spectral modulation of 2MASS 1821, 2MASS 0136, and PSO 318.5-22. Modeling the spectral bright-to-faint ratio of the modulation of 2MASS 1821, 2MASS 0136, and PSO 318.5-22 shows that most spectral characteristics can be reproduced by temperature variations alone. Furthermore, the approximately anti-correlated variability between different wavelengths can be easily interpreted as a change in the temperature gradient in the atmosphere which is the consequence we expect from CO/CH4 radiative convection to explain the L/T transition. The deviation from an exact anti-correlation could then be interpreted as a phase shift similar to the hot-spot shift a different bandpasses in the atmosphere of hot Jupiters. Our results suggest that the rotational spectral modulation from cloud-opacity and temperature variations are degenerate. The detection of direct cloud spectral signatures, e.g. the silicate absorption feature at 10 um, would help to confirm the presence of clouds and their contribution to spectral modulations. Future studies looking at the differences in the spectral modulation of objects with and without the silicate absorption feature may give us some insight on how to distinguish cloud-opacity fluctuations from temperature fluctuations. △ Less

Submitted 14 September, 2020; originally announced September 2020.

Comments: accepted in A&A

Journal ref: A&A 643, A23 (2020)

Direct characterization of young giant exoplanets at high spectral resolution by coupling SPHERE and CRIRES+

Authors: G. P. P. L. Otten, A. Vigan, E. Muslimov, M. N'Diaye, E. Choquet, U. Seemann, K. Dohlen, M. Houllé, P. Cristofari, M. W. Phillips, Y. Charles, I. Baraffe, J. -L. Beuzit, A. Costille, R. Dorn, M. El Morsy, M. Kasper, M. Lopez, C. Mordasini, R. Pourcelot, A. Reiners, J. -F. Sauvage

Abstract: Studies of atmospheres of directly imaged exoplanets with high-resolution spectrographs have shown that their characterization is predominantly limited by noise on the stellar halo at the location of the studied exoplanet. An instrumental combination of high-contrast imaging and high spectral resolution that suppresses this noise and resolves the spectral lines can therefore yield higher quality s… ▽ More Studies of atmospheres of directly imaged exoplanets with high-resolution spectrographs have shown that their characterization is predominantly limited by noise on the stellar halo at the location of the studied exoplanet. An instrumental combination of high-contrast imaging and high spectral resolution that suppresses this noise and resolves the spectral lines can therefore yield higher quality spectra. We study the performance of the proposed HiRISE fiber coupling between the SPHERE and CRIRES+ at the VLT for spectral characterization of directly imaged planets. Using end-to-end simulations of HiRISE we determine the S/N of the detection of molecular species for known exoplanets in $H$ and $K$ bands, and compare them to CRIRES+. We investigate the ultimate detection limits of HiRISE as a function of stellar magnitude, and we quantify the impact of different coronagraphs and of the system transmission. We find that HiRISE largely outperforms CRIRES+ for companions around bright hosts like $β$ Pic or 51 Eri. For an $H=3.5$ host, we observe a gain of a factor of up to 16 in observing time with HiRISE to reach the same S/N on a companion at 200 mas. More generally, HiRISE provides better performance than CRIRES+ in two-hour integration times between 50-350 mas for hosts with $H<8.5$ and between 50-700 mas for $H<7$. For fainter hosts like PDS 70 and HIP 65426, no significant improvements are observed. We find that using no coronagraph yields the best S/N when characterizing known exoplanets due to higher transmission and fiber-based starlight suppression. We demonstrate that the overall transmission of the system is in fact the main driver of performance. Finally, we show that HiRISE outperforms the best detection limits of SPHERE for bright stars, opening major possibilities for the characterization of future planetary companions detected by other techniques. △ Less

Submitted 10 December, 2020; v1 submitted 3 September, 2020; originally announced September 2020.

Comments: 21 pages, 17 figures. 2 tables. Originally accepted for publication in A&A. Revised version after error in sky background was discovered (too high) and corrected in proofing stage. Revisions made in Abstract, Sections 3-6 and Figures 6-12,C1,C2 that address the changes in performance. Resubmitted to A&A

The Parallax of VHS J1256-1257 from CFHT and Pan-STARRS 1

Authors: Trent J. Dupuy, Michael C. Liu, Eugene A. Magnier, William M. J. Best, Isabelle Baraffe, Gilles Chabrier, Thierry Forveille, Stanimir A. Metchev, Pascal Tremblin

Abstract: We present new parallax measurements from the CFHT Infrared Parallax Program and the Pan-STARRS 3$π$ Steradian Survey for the young ($\approx150-300$ Myr) triple system VHS J125601.92$-$125723.9. This system is composed of a nearly equal-flux binary ("AB") and a wide, possibly planetary-mass companion ("b"). The system's published parallactic distance ($12.7\pm1.0$ pc) implies absolute magnitudes… ▽ More We present new parallax measurements from the CFHT Infrared Parallax Program and the Pan-STARRS 3$π$ Steradian Survey for the young ($\approx150-300$ Myr) triple system VHS J125601.92$-$125723.9. This system is composed of a nearly equal-flux binary ("AB") and a wide, possibly planetary-mass companion ("b"). The system's published parallactic distance ($12.7\pm1.0$ pc) implies absolute magnitudes unusually faint compared to known young objects and is in tension with the spectrophotometric distance for the central binary ($17.2\pm2.6$ pc). Our CFHT and Pan-STARRS parallaxes are consistent, and the more precise CFHT result places VHS J1256-1257 at $22.2^{+1.1}_{-1.2}$ pc. Our new distance results in higher values for the companion's mass ($19\pm5$ M$_{\rm Jup}$) and temperature ($1240\pm50$ K), and also brings the absolute magnitudes of all three components into better agreement with known young objects. △ Less

Submitted 10 April, 2020; originally announced April 2020.

Comments: Research Notes of the American Astronomical Society, in press. Arxiv version has additional information about the astrometry (text & figure)

A new set of atmosphere and evolution models for cool T-Y brown dwarfs and giant exoplanets

Authors: Mark W. Phillips, Pascal Tremblin, Isabelle Baraffe, Gilles Chabrier, Nicole F. Allard, Fernand Spiegelman, Jayesh M. Goyal, Ben Drummond, Eric Hebrard

Abstract: We present a new set of solar metallicity atmosphere and evolutionary models for very cool brown dwarfs and self-luminous giant exoplanets, which we term ATMO 2020. Atmosphere models are generated with our state-of-the-art 1D radiative-convective equilibrium code ATMO, and are used as surface boundary conditions to calculate the interior structure and evolution of… ▽ More We present a new set of solar metallicity atmosphere and evolutionary models for very cool brown dwarfs and self-luminous giant exoplanets, which we term ATMO 2020. Atmosphere models are generated with our state-of-the-art 1D radiative-convective equilibrium code ATMO, and are used as surface boundary conditions to calculate the interior structure and evolution of $0.001-0.075\,\mathrm{M_{\odot}}$ objects. Our models include several key improvements to the input physics used in previous models available in the literature. Most notably, the use of a new H-He equation of state including ab initio quantum molecular dynamics calculations has raised the mass by $\sim1-2\%$ at the stellar-substellar boundary and has altered the cooling tracks around the hydrogen and deuterium burning minimum masses. A second key improvement concerns updated molecular opacities in our atmosphere model ATMO, which now contains significantly more line transitions required to accurately capture the opacity in these hot atmospheres. This leads to warmer atmospheric temperature structures, further changing the cooling curves and predicted emission spectra of substellar objects. We present significant improvement for the treatment of the collisionally broadened potassium resonance doublet, and highlight the importance of these lines in shaping the red-optical and near-infrared spectrum of brown dwarfs. We generate three different grids of model simulations, one using equilibrium chemistry and two using non-equilibrium chemistry due to vertical mixing, all three computed self-consistently with the pressure-temperature structure of the atmosphere. We show the impact of vertical mixing on emission spectra and in colour-magnitude diagrams, highlighting how the $3.5-5.5\,\mathrm{μm}$ flux window can be used to calibrate vertical mixing in cool T-Y spectral type objects. △ Less

Submitted 30 March, 2020; originally announced March 2020.

Comments: 21 pages, 18 figures, 1 table. Accepted for publication in A&A. Models available at http://opendata.erc-atmo.eu and http://perso.ens-lyon.fr/isabelle.baraffe/ATMO2020/

Journal ref: A&A 637, A38 (2020)

Comparison of two- and three-dimensional compressible convection in a pre-main sequence star

Authors: J. Pratt, I. Baraffe, T. Goffrey, C. Geroux, T. Constantino, D. Folini, R. Walder

Abstract: Extending our recent studies of two-dimensional stellar convection to 3D, we compare three-dimensional hydrodynamic simulations to identically set-up two-dimensional simulations, for a realistic pre-main sequence star. We compare statistical quantities related to convective flows including: average velocity, vorticity, local enstrophy, and penetration depth beneath a convection zone. These statist… ▽ More Extending our recent studies of two-dimensional stellar convection to 3D, we compare three-dimensional hydrodynamic simulations to identically set-up two-dimensional simulations, for a realistic pre-main sequence star. We compare statistical quantities related to convective flows including: average velocity, vorticity, local enstrophy, and penetration depth beneath a convection zone. These statistics are produced during stationary, steady-state compressible convection in the star's convection zone. Our simulations with the MUSIC code confirm the common result that two-dimensional simulations of stellar convection have a higher magnitude of velocity on average than three-dimensional simulations. Boundary conditions and the extent of the spherical shell can affect the magnitude and variability of convective velocities. The difference between 2D and 3D velocities is dependent on these background points; in our simulations this can have an effect as large as the difference resulting from the dimensionality of the simulation. Nevertheless, radial velocities near the convective boundary are comparable in our 2D and 3D simulations. The average local enstrophy of the flow is lower for two-dimensional simulations than for three-dimensional simulations, indicating a different shape and structuring of 3D stellar convection. We perform a statistical analysis of the depth of convective penetration below the convection zone, using the model proposed in our recent study (Pratt et al. 2017). Here we analyze the convective penetration in three dimensional simulations, and compare the results to identically set-up 2D simulations. In 3D the penetration depth is as large as the penetration depth calculated from 2D simulations. △ Less

Submitted 10 March, 2020; originally announced March 2020.

Comments: 13 pages, 8 figures

Journal ref: A&A 638, A15 (2020)

Idealised simulations of the deep atmosphere of hot jupiters: Deep, hot, adiabats as a robust solution to the radius inflation problem

Authors: F. Sainsbury-Martinez, P. Wang, S. Fromang, P. Tremblin, T. Dubos, Y. Meurdesoif, A. Spiga, J. Leconte, I. Baraffe, G. Chabrier, N. Mayne, B. Drummond, F. Debras

Abstract: Context: The anomalously large radii of hot Jupiters has long been a mystery. However, by combining both theoretical arguments and 2D models, a recent study has suggested that the vertical advection of potential temperature leads to an adiabatic temperature profile in the deep atmosphere hotter than the profile obtained with standard 1D models. Aims: In order to confirm the viability of that scena… ▽ More Context: The anomalously large radii of hot Jupiters has long been a mystery. However, by combining both theoretical arguments and 2D models, a recent study has suggested that the vertical advection of potential temperature leads to an adiabatic temperature profile in the deep atmosphere hotter than the profile obtained with standard 1D models. Aims: In order to confirm the viability of that scenario, we extend this investigation to three dimensional, time-dependent, models. Methods: We use a 3D GCM, DYNAMICO to perform a series of calculations designed to explore the formation and structure of the driving atmospheric circulations, and detail how it responds to changes in both upper and deep atmospheric forcing. Results: In agreement with the previous, 2D, study, we find that a hot adiabat is the natural outcome of the long-term evolution of the deep atmosphere. Integration times of order $1500$ years are needed for that adiabat to emerge from an isothermal atmosphere, explaining why it has not been found in previous hot Jupiter studies. Models initialised from a hotter deep atmosphere tend to evolve faster toward the same final state. We also find that the deep adiabat is stable against low-levels of deep heating and cooling, as long as the Newtonian cooling time-scale is longer than $\sim 3000$ years at $200$ bar. Conclusions: We conclude that the steady-state vertical advection of potential temperature by deep atmospheric circulations constitutes a robust mechanism to explain hot Jupiter inflated radii. We suggest that future studies of hot Jupiters are evolved for a longer time than currently done, and, when possible, include models initialised with a hot deep adiabat. We stress that this mechanism stems from the advection of entropy by irradiation induced mass flows and does not require (finely tuned) dissipative process, in contrast with most previously suggested scenarios. △ Less

Submitted 15 November, 2019; originally announced November 2019.

Comments: 13 pages, 12 figures. Accepted for publication in A&A

Journal ref: A&A 632, A114 (2019)

The Acceleration of Superrotation in Simulated Hot Jupiter Atmospheres

Authors: Florian Debras, Nathan Mayne, Isabelle Baraffe, Etienne Jaupart, Pierre Mourier, Guillaume Laibe, Tom Goffrey, John Thuburn

Abstract: Context. Atmospheric superrotating flows at the equator are an almost ubiquitous result of simulations of hot Jupiters, and a theory explaining how this zonally coherent flow reaches an equilibrium has been developed in the literature. However, this understanding relies on the existence of either an initial superrotating or a sheared flow, coupled with a slow evolution such that a linear steady st… ▽ More Context. Atmospheric superrotating flows at the equator are an almost ubiquitous result of simulations of hot Jupiters, and a theory explaining how this zonally coherent flow reaches an equilibrium has been developed in the literature. However, this understanding relies on the existence of either an initial superrotating or a sheared flow, coupled with a slow evolution such that a linear steady state can be reached. Aims. A consistent physical understanding of superrotation is needed for arbitrary drag and radiative timescales, and the relevance of considering linear steady states needs to be assessed. Methods. We obtain an analytical expression for the structure, frequency and decay rate of propagating waves in hot Jupiter atmospheres around a state at rest in the 2D shallow water beta plane limit. We solve this expression numerically and confirm the robustness of our results with a 3D linear wave algorithm. We then compare with 3D simulations of hot Jupiter atmospheres and study the non linear momentum fluxes. Results. We show that under strong day night heating the dynamics does not transit through a linear steady state when starting from an initial atmosphere in solid body rotation. We further show that non linear effects favour the initial spin up of superrotation and that the acceleration due to the vertical component of the eddy momentum flux is critical to the initial development of superrotation. Conclusions. Overall, we describe the initial phases of the acceleration of superrotation, including consideration of differing radiative and drag timescales, and conclude that eddy-momentum driven superrotating equatorial jets are robust, physical phenomena in simulations of hot Jupiter atmospheres. △ Less

Submitted 8 November, 2019; originally announced November 2019.

Comments: 28 pages, 20 pages of text - 8 of appendices, 9 figures in text - 6 in appendices

Eigenvectors, Circulation and Linear Instabilities for Planetary Science in 3 Dimensions (ECLIPS3D)

Authors: Florian Debras, Nathan Mayne, Isabelle Baraffe, Tom Goffrey, John Thuburn

Abstract: Context. The study of linear waves and instabilities is necessary to understand the physical evolution of an atmosphere, and can provide physical interpretation of the complex flows found in simulations performed using Global Circulation Models (GCM). In particular, the acceleration of superrotating flow at the equator of hot Jupiters has mostly been studied under several simplifying assumptions,… ▽ More Context. The study of linear waves and instabilities is necessary to understand the physical evolution of an atmosphere, and can provide physical interpretation of the complex flows found in simulations performed using Global Circulation Models (GCM). In particular, the acceleration of superrotating flow at the equator of hot Jupiters has mostly been studied under several simplifying assumptions, the relaxing of which may impact final results. Aims. We develop and benchmark a publicly available algorithm to identify the eigenmodes of an atmosphere around any initial steady state. We also solve for linear steady states. Methods. We linearise the hydrodynamical equations of a planetary atmosphere in a steady state with arbitrary velocities and thermal profile. We then discretise the linearised equations on an appropriate staggered grid, and solve for eigenvectors and linear steady solutions with the use of a parallel library for linear algebra: ScaLAPACK. We also implement a posteriori calculation of an energy equation in order to obtain more information on the underlying physics of the mode. Results. Our code is benchmarked against classical wave and instability test cases in multiple geometries. The steady linear circulation calculations also reproduce expected results for the atmosphere of hot Jupiters. We finally show the robustness of our energy equation, and its power to obtain physical insight into the modes. Conclusions. We have developed and benchmarked a code for the study of linear processes in planetary atmospheres, with an arbitrary steady state. The calculation of an a posteriori energy equation provides both increased robustness and physical meaning to the obtained eigenmodes. This code can be applied to various problems, and notably to further study the initial spin up of superrotation of GCM simulations of hot Jupiter. △ Less

Submitted 9 September, 2019; originally announced September 2019.

Comments: 18 pages (6 of appendix), 9 figures

Journal ref: A&A 631, A36 (2019)

A well-balanced scheme for the simulation tool-kit A-MaZe: implementation, tests, and first applications to stellar structure

Authors: M. V. Popov, R. Walder, D. Folini, T. Goffrey, I. Baraffe, T. Constantino, C. Geroux, J. Pratt, M. Viallet

Abstract: Characterizing stellar convection in multiple dimensions is a topic at the forefront of stellar astrophysics. Numerical simulations are an essential tool for this task. We present an extension of the existing numerical tool-kit A-MaZe that enables such simulations of stratified flows in a gravitational field. The finite-volume based, cell-centered, and time-explicit hydrodynamics solver of A-MaZe… ▽ More Characterizing stellar convection in multiple dimensions is a topic at the forefront of stellar astrophysics. Numerical simulations are an essential tool for this task. We present an extension of the existing numerical tool-kit A-MaZe that enables such simulations of stratified flows in a gravitational field. The finite-volume based, cell-centered, and time-explicit hydrodynamics solver of A-MaZe was extended such that the scheme is now well-balanced in both momentum and energy. The algorithm maintains an initially static balance between gravity and pressure to machine precision. Quasi-stationary convection in slab-geometry preserves gas energy (internal plus kinetic) on average despite strong local up- and down-drafts. By contrast, a more standard numerical scheme is demonstrated to result in substantial gains of energy within a short time on purely numerical grounds. The test is further used to point out the role of dimensionality, viscosity, and Rayleigh number for compressible convection. Applications to a young sun in 2D and 3D, covering a part of the inner radiative zone as well as the outer convective zone, demonstrate that the scheme meets its initial design goal. Comparison with results obtained for a physically identical setup with a time-implicit code show qualitative agreement. △ Less

Submitted 5 September, 2019; originally announced September 2019.

Comments: 12 pages, 8 figures, accepted 30/08/2019 for publication by Astronomy & Astrophysics

Journal ref: A&A 630, A129 (2019)

WISE J072003.20-084651.2B Is A Massive T Dwarf

Authors: Trent J. Dupuy, Michael C. Liu, William M. J. Best, Andrew W. Mann, Michael A. Tucker, Zhoujian Zhang, Isabelle Baraffe, Gilles Chabrier, Thierry Forveille, Stanimir A. Metchev, Pascal Tremblin, Aaron Do, Anna V. Payne, B. J. Shappee, Charlotte Z. Bond, Sylvain Cetre, Mark Chun, Jacques-Robert Delorme, Nemanja Jovanovic, Scott Lilley, Dimitri Mawet, Sam Ragland, Ed Wetherell, Peter Wizinowich

Abstract: We present individual dynamical masses for the nearby M9.5+T5.5 binary WISE J072003.20$-$084651.2AB, a.k.a. Scholz's star. Combining high-precision CFHT/WIRCam photocenter astrometry and Keck adaptive optics resolved imaging, we measure the first high-quality parallactic distance ($6.80_{-0.06}^{+0.05}$ pc) and orbit ($8.06_{-0.25}^{+0.24}$ yr period) for this system composed of a low-mass star an… ▽ More We present individual dynamical masses for the nearby M9.5+T5.5 binary WISE J072003.20$-$084651.2AB, a.k.a. Scholz's star. Combining high-precision CFHT/WIRCam photocenter astrometry and Keck adaptive optics resolved imaging, we measure the first high-quality parallactic distance ($6.80_{-0.06}^{+0.05}$ pc) and orbit ($8.06_{-0.25}^{+0.24}$ yr period) for this system composed of a low-mass star and brown dwarf. We find a moderately eccentric orbit ($e = 0.240_{-0.010}^{+0.009}$), incompatible with previous work based on less data, and dynamical masses of $99\pm6$ $M_{\rm Jup}$ and $66\pm4$ $M_{\rm Jup}$ for the two components. The primary mass is marginally inconsistent (2.1$σ$) with the empirical mass$-$magnitude$-$metallicity relation and models of main-sequence stars. The relatively high mass of the cold ($T_{\rm eff} = 1250\pm40$ K) brown dwarf companion indicates an age older than a few Gyr, in accord with age estimates for the primary star, and is consistent with our recent estimate of $\approx$70 $M_{\rm Jup}$ for the stellar/substellar boundary among the field population. Our improved parallax and proper motion, as well as an orbit-corrected system velocity, improve the accuracy of the system's close encounter with the solar system by an order of magnitude. WISE J0720$-$0846AB passed within $68.7\pm2.0$ kAU of the Sun $80.5\pm0.7$ kyr ago, passing through the outer Oort cloud where comets can have stable orbits. △ Less

Submitted 19 August, 2019; originally announced August 2019.

Comments: accepted to AJ

ESA Voyage 2050 White Paper: Detecting life outside our solar system with a large high-contrast-imaging mission

Authors: Ignas Snellen, Simon Albrecht, Guillem Anglada-Escude, Isabelle Baraffe, Pierre Baudoz, Willy Benz, Jean-Luc Beuzit, Beth Biller, Jayne Birkby, Anthony Boccaletti, Roy van Boekel, Jos de Boer, Matteo Brogi, Lars Buchhave, Ludmila Carone, Mark Claire, Riccardo Claudi, Brice-Olivier Demory, Jean-Michel Desert, Silvano Desidera, Scott Gaudi, Raffaele Gratton, Michael Gillon, John Lee Grenfell, Olivier Guyon , et al. (42 additional authors not shown)

Abstract: In this white paper, we recommend the European Space Agency plays a proactive role in developing a global collaborative effort to construct a large high-contrast imaging space telescope, e.g. as currently under study by NASA. Such a mission will be needed to characterize a sizable sample of temperate Earth-like planets in the habitable zones of nearby Sun-like stars and to search for extraterrestr… ▽ More In this white paper, we recommend the European Space Agency plays a proactive role in developing a global collaborative effort to construct a large high-contrast imaging space telescope, e.g. as currently under study by NASA. Such a mission will be needed to characterize a sizable sample of temperate Earth-like planets in the habitable zones of nearby Sun-like stars and to search for extraterrestrial biological activity. We provide an overview of relevant European expertise, and advocate ESA to start a technology development program towards detecting life outside the Solar system. △ Less

Submitted 5 August, 2019; originally announced August 2019.

Comments: White paper for ESA Voyage 2050; 24 pages

Hunting for ancient brown dwarfs: the developing field of brown dwarfs in globular clusters

Authors: Ilaria Caiazzo, Adam Burgasser, Jon M. Rees, France Allard, Andrea Dieball, Jeremy Heyl, Harvey Richer, Isabelle Baraffe, Christian Knigge

Abstract: The detection of brown dwarfs in globular star clusters will allow us to break the degeneracies in age, mass and composition that affect our current models, and therefore to constrain the physics of their atmospheres and interiors. Furthermore, detecting brown dwarfs will help us constrain the properties of the clusters themselves, as they carry information about the clusters' age and dynamics. Gr… ▽ More The detection of brown dwarfs in globular star clusters will allow us to break the degeneracies in age, mass and composition that affect our current models, and therefore to constrain the physics of their atmospheres and interiors. Furthermore, detecting brown dwarfs will help us constrain the properties of the clusters themselves, as they carry information about the clusters' age and dynamics. Great advancements in this field are to be expected in the next ten years, thanks to the extraordinary sensitivity in the infrared of upcoming telescopes like JWST and the ELTs. △ Less

Submitted 15 March, 2019; originally announced March 2019.

Comments: White paper submitted for Astro2020 Decadal Survey. 8 pages, 2 figures

Astro2020 Science White Paper: Fundamental Physics with Brown Dwarfs: The Mass-Radius Relation

Authors: Adam Burgasser, Isabelle Baraffe, Matthew Browning, Adam Burrows, Gilles Chabrier, Michelle Creech-Eakman, Brice Demory, Sergio Dieterich, Jacqueline Faherty, Daniel Huber, Nicolas Lodieu, Peter Plavchan, R. Michael Rich, Didier Saumon, Keivan Stassun, Amaury Triaud, Gerard van Belle, Valerie Van Grootel, Johanna M. Vos, Rakesh Yadav

Abstract: The lowest-mass stars, brown dwarfs and giant exoplanets span a minimum in the mass-radius relationship that probes the fundamental physics of extreme states of matter, magnetism, and fusion. This White Paper outlines scientific opportunities and the necessary resources for modeling and measuring the mass-radius relationship in this regime. The lowest-mass stars, brown dwarfs and giant exoplanets span a minimum in the mass-radius relationship that probes the fundamental physics of extreme states of matter, magnetism, and fusion. This White Paper outlines scientific opportunities and the necessary resources for modeling and measuring the mass-radius relationship in this regime. △ Less

Submitted 11 March, 2019; originally announced March 2019.

Comments: 7 pages, submitted to Astro2020 Science White Paper call

Thermo-compositional diabatic convection in the atmospheres of brown dwarfs and in Earth's atmosphere and oceans

Authors: P. Tremblin, T. Padioleau, M. Phillips, G. Chabrier, I. Baraffe, S. Fromang, E. Audit, H. Bloch, A. J. Burgasser, B. Drummond, M. Gonzalez, P. Kestener, S. Kokh, P. -O. Lagage, M. Stauffert

Abstract: By generalizing the theory of convection to any type of thermal and compositional source terms (diabatic processes), we show that thermohaline convection in Earth oceans, fingering convection in stellar atmospheres, and moist convection in Earth atmosphere are deriving from the same general diabatic convective instability. We show also that "radiative convection" triggered by CO/CH4 transition wit… ▽ More By generalizing the theory of convection to any type of thermal and compositional source terms (diabatic processes), we show that thermohaline convection in Earth oceans, fingering convection in stellar atmospheres, and moist convection in Earth atmosphere are deriving from the same general diabatic convective instability. We show also that "radiative convection" triggered by CO/CH4 transition with radiative transfer in the atmospheres of brown dwarfs is analog to moist and thermohaline convection. We derive a generalization of the mixing length theory to include the effect of source terms in 1D codes. We show that CO/CH4 radiative convection could significantly reduce the temperature gradient in the atmospheres of brown dwarfs similarly to moist convection in Earth atmosphere thus possibly explaining the reddening in brown-dwarf spectra. By using idealized two-dimensional hydrodynamic simulations in the Ledoux unstable regime, we show that compositional source terms can indeed provoke a reduction of the temperature gradient. The L/T transition could be explained by a bifurcation between the adiabatic and diabatic convective transports and could be seen as a giant cooling crisis: an analog of the boiling crisis in liquid/steam-water convective flows. This mechanism with other chemical transitions could be present in many giant and earth-like exoplanets. The study of the impact of different parameters (effective temperature, compositional changes) on CO/CH4 radiative convection and the analogy with Earth moist and thermohaline convection is opening the possibility to use brown dwarfs to better understand some aspects of the physics at play in the climate of our own planet. △ Less

Submitted 10 February, 2019; originally announced February 2019.

Comments: accepted in ApJ

The Limits of the Primitive Equations of Dynamics for Warm, Slowly Rotating Small Neptunes and Super Earths

Authors: N. J. Mayne, B. Drummond, F. Debras, E. Jaupart, J. Manners, I. A. Boutle, I. Baraffe, K. Kohary

Abstract: We present significant differences in the simulated atmospheric flow for warm, tidally-locked small Neptunes and super Earths (based on a nominal GJ 1214b) when solving the simplified, and commonly used, primitive dynamical equations or the full Navier-Stokes equations. The dominant prograde, superrotating zonal jet is markedly different between the simulations which are performed using practicall… ▽ More We present significant differences in the simulated atmospheric flow for warm, tidally-locked small Neptunes and super Earths (based on a nominal GJ 1214b) when solving the simplified, and commonly used, primitive dynamical equations or the full Navier-Stokes equations. The dominant prograde, superrotating zonal jet is markedly different between the simulations which are performed using practically identical numerical setups, within the same model. The differences arise due to the breakdown of the so-called `shallow-fluid' and traditional approximations, which worsens when rotation rates are slowed, and day-night temperature contrasts are increased. The changes in the zonal advection between simulations solving the full and simplified equations, give rise to significant differences in the atmospheric redistribution of heat, altering the position of the hottest part of the atmosphere and temperature contrast between the day and night sides. The implications for the atmospheric chemistry and, therefore, observations need to be studied with a model including a more detailed treatment of the radiative transfer and chemistry. Small Neptunes and super Earths are extremely abundant and important, potentially bridging the structural properties (mass, radius, composition) of terrestrial and gas giant planets. Our results indicate care is required when interpreting the output of models solving the primitive equations of motion for such planets. △ Less

Submitted 6 December, 2018; originally announced December 2018.

Comments: 26 pages, 11 figures, Accepted for Publication in ApJ

The 3D thermal, dynamical and chemical structure of the atmosphere of HD 189733b: implications of wind-driven chemistry for the emission phase curve

Authors: Benjamin Drummond, Nathan J. Mayne, James Manners, Isabelle Baraffe, Jayesh Goyal, Pascal Tremblin, David K. Sing, Krisztian Kohary

Abstract: In this paper we present three-dimensional atmospheric simulations of the hot Jupiter HD~189733b under two different scenarios: local chemical equilibrium and including advection of the chemistry by the resolved wind. Our model consistently couples the treatment of dynamics, radiative transfer and chemistry, completing the feedback cycle between these three important processes. The effect of wind-… ▽ More In this paper we present three-dimensional atmospheric simulations of the hot Jupiter HD~189733b under two different scenarios: local chemical equilibrium and including advection of the chemistry by the resolved wind. Our model consistently couples the treatment of dynamics, radiative transfer and chemistry, completing the feedback cycle between these three important processes. The effect of wind--driven advection on the chemical composition is qualitatively similar to our previous results for the warmer atmosphere of HD~209458b, found using the same model. However, we find more significant alterations to both the thermal and dynamical structure for the cooler atmosphere of HD~189733b, with changes in both the temperature and wind velocities reaching $\sim10\%$. We also present the contribution function, diagnosed from our simulations, and show that wind--driven chemistry has a significant impact on its three--dimensional structure, particularly for regions where methane is an important absorber. Finally, we present emission phase curves from our simulations and show the significant effect of wind--driven chemistry on the thermal emission, particularly within the 3.6 \textmu m Spitzer/IRAC channel. △ Less

Submitted 23 October, 2018; originally announced October 2018.

Comments: Accepted in ApJ

A closer look at the transition between fully convective and partly radiative low mass stars

Authors: Isabelle Baraffe, Gilles Chabrier

Abstract: Recently, Jao et al. (2018) discovered a gap in the mid-M dwarfs main sequence revealed by the analysis of Gaia data Release 2. They suggested the feature is linked to the onset of full convection in M dwarfs. Following the announcement of this discovery, MacDonald & Gizis (2018) proposed an explanation based on standard stellar evolution models. In this paper we re-examine the explanation suggest… ▽ More Recently, Jao et al. (2018) discovered a gap in the mid-M dwarfs main sequence revealed by the analysis of Gaia data Release 2. They suggested the feature is linked to the onset of full convection in M dwarfs. Following the announcement of this discovery, MacDonald & Gizis (2018) proposed an explanation based on standard stellar evolution models. In this paper we re-examine the explanation suggested by MacDonald & Gizis (2018). We confirm that nuclear burning and mixing process of $^3$He provide the best explanation for the observed feature. We also find that a change in the energy transport from convection to radiation does not induce structural changes that could be visible. Regarding the very details of the process, however, we disagree with MacDonald & Gizis (2018) and propose a different explanation. △ Less

Submitted 19 September, 2018; originally announced September 2018.

Comments: 5 pages, 6 figures, accepted for publication in Astronomy & Astrophysics

Journal ref: A&A 619, A177 (2018)

Significant uncertainties from calibrating overshooting with eclipsing binary systems

Authors: Thomas Constantino, Isabelle Baraffe

Abstract: The precise measurement of the masses and radii of stars in eclipsing binary systems provides a window into uncertain processes in stellar evolution, especially mixing at convective boundaries. Recently, these data have been used to calibrate models of convective overshooting in the cores of main sequence stars. In this study we have used a small representative sample of eclipsing binary stars wit… ▽ More The precise measurement of the masses and radii of stars in eclipsing binary systems provides a window into uncertain processes in stellar evolution, especially mixing at convective boundaries. Recently, these data have been used to calibrate models of convective overshooting in the cores of main sequence stars. In this study we have used a small representative sample of eclipsing binary stars with $1.25 \leq M/\text{M}_\odot < 4.2$ to test how precisely this method can constrain the overshooting and whether the data support a universal stellar mass--overshooting relation. We do not recover the previously reported stellar mass dependence for the extent of overshooting and in each case we find there is a substantial amount of uncertainty, that is, the same binary pair can be matched by models with different amounts of overshooting. Models with a moderate overshooting parameter $0.013 \leq f_\text{os} \leq 0.014$ (using the scheme from Herwig et al. 1997) are consistent with all eight systems studied. Generally, a much larger range of $f_\text{os}$ is suitable for individual systems. In the case of main sequence and early post-main sequence stars, large changes in the amount of overshooting have little effect on the radius and effective temperature, and therefore the method is of extremely limited utility. △ Less

Submitted 28 August, 2018; v1 submitted 10 August, 2018; originally announced August 2018.

Comments: Accepted for publication in A&A

Journal ref: A&A 618, A177 (2018)

Bringing high-spectral resolution to VLT/SPHERE with a fibre coupling to VLT/CRIRES+

Authors: A. Vigan, G. P. P. L. Otten, E. Muslimov, K. Dohlen, M. W. Phillips, U. Seemann, J. -L. Beuzit, R. Dorn, M. Kasper, D. Mouillet, I. Baraffe, A. Reiners

Abstract: Atmospheric composition provides essential markers of the most fundamental properties of giant exoplanets, such as their formation mechanism or internal structure. New-generation exoplanet imagers, like VLT/SPHERE or Gemini/GPI, have been designed to achieve very high contrast (>15 mag) at small angular separations ($<$0.5\as) for the detection of young giant planets in the near-infrared, but they… ▽ More Atmospheric composition provides essential markers of the most fundamental properties of giant exoplanets, such as their formation mechanism or internal structure. New-generation exoplanet imagers, like VLT/SPHERE or Gemini/GPI, have been designed to achieve very high contrast (>15 mag) at small angular separations ($<$0.5\as) for the detection of young giant planets in the near-infrared, but they only provide very low spectral resolutions ($R<100$) for their characterization. High-dispersion spectroscopy at resolutions up to $10^5$ is one of the most promising pathways for the detailed characterization of exoplanets, but it is currently out of reach for most directly imaged exoplanets because current high-dispersion spectrographs in the near-infrared lack coronagraphs to attenuate the stellar signal and the spatial resolution necessary to resolve the planet. Project HiRISE (High-Resolution Imaging and Spectroscopy of Exoplanets) ambitions to develop a demonstrator that will combine the capabilities of two flagship instruments installed on the ESO Very Large Telescope, the high-contrast exoplanet imager SPHERE and the high-resolution spectrograph CRIRES+, with the goal of answering fundamental questions on the formation, composition and evolution of young planets. In this work, we will present the project, the first set of realistic simulations and the preliminary design of the fiber injection unit that will be implemented in SPHERE. △ Less

Submitted 27 June, 2018; originally announced June 2018.

Comments: 18 pages, 12 figures. Proceeding of the SPIE Astronomical Telescopes + Instrumentation Conference, 10 - 15 June 2018

Observable signatures of wind--driven chemistry with a fully consistent three dimensional radiative hydrodynamics model of HD 209458b

Authors: Benjamin Drummond, N. J. Mayne, James Manners, Aarynn L. Carter, Ian A. Boutle, Isabelle Baraffe, Eric Hebrard, Pascal Tremblin, David K. Sing, David S. Amundsen, Dave Acreman

Abstract: We present a study of the effect of wind-driven advection on the chemical composition of hot Jupiter atmospheres using a fully-consistent 3D hydrodynamics, chemistry and radiative transfer code, the Met Office Unified Model (UM). Chemical modelling of exoplanet atmospheres has primarily been restricted to 1D models that cannot account for 3D dynamical processes. In this work we couple a chemical r… ▽ More We present a study of the effect of wind-driven advection on the chemical composition of hot Jupiter atmospheres using a fully-consistent 3D hydrodynamics, chemistry and radiative transfer code, the Met Office Unified Model (UM). Chemical modelling of exoplanet atmospheres has primarily been restricted to 1D models that cannot account for 3D dynamical processes. In this work we couple a chemical relaxation scheme to the UM to account for the chemical interconversion of methane and carbon monoxide. This is done consistently with the radiative transfer meaning that departures from chemical equilibrium are included in the heating rates (and emission) and hence complete the feedback between the dynamics, thermal structure and chemical composition. In this letter we simulate the well studied atmosphere of HD~209458b. We find that the combined effect of horizontal and vertical advection leads to an increase in the methane abundance by several orders of magnitude; directly opposite to the trend found in previous works. Our results demonstrate the need to include 3D effects when considering the chemistry of hot Jupiter atmospheres. We calculate transmission and emission spectra, as well as the emission phase curve, from our simulations. We conclude that gas-phase non-equilibrium chemistry is unlikely to explain the model-observation discrepancy in the 4.5\,{\textmu m} {\it Spitzer}/IRAC channel. However, we highlight other spectral regions, observable with the James Webb Space Telescope, where signatures of wind-driven chemistry are more prominant. △ Less

Submitted 26 February, 2018; originally announced February 2018.

Comments: Accepted for publication in ApJL

The effect of metallicity on the atmospheres of exoplanets with fully coupled 3D hydrodynamics, equilibrium chemistry, and radiative transfer

Authors: Benjamin Drummond, N. J. Mayne, Isabelle Baraffe, Pascal Tremblin, James Manners, David S. Amundsen, Jayesh Goyal, Dave Acreman

Abstract: In this work we have performed a series of simulations of the atmosphere of GJ~1214b assuming different metallicities using the Met Office Unified Model (UM). The UM is a general circulation model (GCM) that solves the deep, non-hydrostatic equations of motion and uses a flexible and accurate radiative transfer scheme, based on the two-stream and correlated-$k$ approximations, to calculate the hea… ▽ More In this work we have performed a series of simulations of the atmosphere of GJ~1214b assuming different metallicities using the Met Office Unified Model (UM). The UM is a general circulation model (GCM) that solves the deep, non-hydrostatic equations of motion and uses a flexible and accurate radiative transfer scheme, based on the two-stream and correlated-$k$ approximations, to calculate the heating rates. In this work we consistently couple a well-tested Gibbs energy minimisation scheme to solve for the chemical equilibrium abundances locally in each grid cell for a general set of elemental abundances, further improving the flexibility and accuracy of the model. As the metallicity of the atmosphere is increased we find significant changes in the dynamical and thermal structure, with subsequent implications for the simulated phase curve. The trends that we find are qualitatively consistent with previous works, though with quantitative differences. We investigate in detail the effect of increasing the metallicity by splitting the mechanism into constituents, involving the mean molecular weight, the heat capacity and the opacities. We find the opacity effect to be the dominant mechanism in altering the circulation and thermal structure. This result highlights the importance of accurately computing the opacities and radiative transfer in 3D GCMs. △ Less

Submitted 3 January, 2018; originally announced January 2018.

Comments: Accepted for publication in Astronomy and Astrophysics

A library of ATMO forward model transmission spectra for hot Jupiter exoplanets

Authors: Jayesh M. Goyal, Nathan Mayne, David K. Sing, Benjamin Drummond, Pascal Tremblin, David S. Amundsen, Thomas Evans, Aarynn L. Carter, Jessica Spake, Isabelle Baraffe, Nikolay Nikolov, James Manners, Gilles Chabrier, Eric Hebrard

Abstract: We present a grid of forward model transmission spectra, adopting an isothermal temperature-pressure profile, alongside corresponding equilibrium chemical abundances for 117 observationally significant hot exoplanets (Equilibrium Temperatures of 547-2710 K). This model grid has been developed using a 1D radiative-convective-chemical equilibrium model termed ATMO, with up-to-date high temperature o… ▽ More We present a grid of forward model transmission spectra, adopting an isothermal temperature-pressure profile, alongside corresponding equilibrium chemical abundances for 117 observationally significant hot exoplanets (Equilibrium Temperatures of 547-2710 K). This model grid has been developed using a 1D radiative-convective-chemical equilibrium model termed ATMO, with up-to-date high temperature opacities. We present an interpretation of observations of ten exoplanets, including best fit parameters and $χ^{2}$ maps. In agreement with previous works, we find a continuum from clear to hazy/cloudy atmospheres for this sample of hot Jupiters. The data for all the 10 planets are consistent with sub-solar to solar C/O ratio, 0.005 to 10 times solar metallicity and water rather than a methane dominated infrared spectra. We then explore the range of simulated atmospheric spectra for different exoplanets, based on characteristics such as temperature, metallicity, C/O-ratio, haziness and cloudiness. We find a transition value for the metallicity between 10 and 50 times solar, which leads to substantial changes in the transmission spectra. We also find a transition value of C/O ratio, from water to carbon species dominated infrared spectra, as found by previous works, revealing a temperature dependence of this transition point ranging from $\sim$0.56 to $\sim$1-1.3 for equilibrium temperatures from $\sim$900 to $\sim$2600 K. We highlight the potential of the spectral features of HCN and C$_2$H$_2$ to constrain the metallicities and C/O ratios of planets, using JWST observations. Finally, our entire grid ($\sim$460,000 simulations) is publicly available and can be used directly with the JWST simulator PandExo for planning observations. △ Less

Submitted 29 July, 2020; v1 submitted 27 October, 2017; originally announced October 2017.

Comments: No changes in replacement, main paper now made visible on arXiv as earlier the published Erratum associated with the paper was instead visible. 34 pages, 28 figures. Accepted for Publication in MNRAS. Full grid of model transmission spectra and chemical abundances are available here, https://drive.google.com/drive/folders/1tvlgSWyEAA0cX_yxnA5URD4Zo3Zj4GcH

Journal ref: Monthly Notices of the Royal Astronomical Society, Volume 474, Issue 4, March 2018, Pages 5158-5185

Cloudless atmospheres for young low-gravity substellar objects

Authors: P. Tremblin, G. Chabrier, I. Baraffe, Michael. C. Liu, E. A. Magnier, P. -O. Lagage, C. Alves de Oliveira, A. J. Burgasser, D. S. Amundsen, B. Drummond

Abstract: Atmospheric modeling of low-gravity (VL-G) young brown dwarfs remains a challenge. The presence of very thick clouds has been suggested because of their extremely red near-infrared (NIR) spectra, but no cloud models provide a good fit to the data with a radius compatible with evolutionary models for these objects. We show that cloudless atmospheres assuming a temperature gradient reduction caused… ▽ More Atmospheric modeling of low-gravity (VL-G) young brown dwarfs remains a challenge. The presence of very thick clouds has been suggested because of their extremely red near-infrared (NIR) spectra, but no cloud models provide a good fit to the data with a radius compatible with evolutionary models for these objects. We show that cloudless atmospheres assuming a temperature gradient reduction caused by fingering convection provides a very good model to match the observed VL-G NIR spectra. The sequence of extremely red colors in the NIR for atmospheres with effective temperature from ~2000 K down to ~1200 K is very well reproduced with predicted radii typical of young low-gravity objects. Future observations with NIRSPEC and MIRI on the James Webb Space Telescope (JWST) will provide more constrains in the mid-infrared, helping to confirm/refute whether or not the NIR reddening is caused by fingering convection. We suggest that the presence/absence of clouds will be directly determined by the silicate absorption features that can be observed with MIRI. JWST will therefore be able to better characterize the atmosphere of these hot young brown dwarfs and their low-gravity exoplanet analogues. △ Less

Submitted 7 October, 2017; originally announced October 2017.

Comments: Accepted in ApJ

Lithium depletion in solar-like stars: effect of overshooting based on realistic multi-dimensional simulations

Authors: I. Baraffe, J. Pratt, T. Goffrey, T. Constantino, D. Folini, M. V. Popov, R. Walder, M. Viallet

Abstract: We study lithium depletion in low-mass and solar-like stars as a function of time, using a new diffusion coefficient describing extra-mixing taking place at the bottom of a convective envelope. This new form is motivated by multi-dimensional fully compressible, time implicit hydrodynamic simulations performed with the MUSIC code. Intermittent convective mixing at the convective boundary in a star… ▽ More We study lithium depletion in low-mass and solar-like stars as a function of time, using a new diffusion coefficient describing extra-mixing taking place at the bottom of a convective envelope. This new form is motivated by multi-dimensional fully compressible, time implicit hydrodynamic simulations performed with the MUSIC code. Intermittent convective mixing at the convective boundary in a star can be modeled using extreme value theory, a statistical analysis frequently used for finance, meteorology, and environmental science. In this letter, we implement this statistical diffusion coefficient in a one-dimensional stellar evolution code, using parameters calibrated from multi-dimensional hydrodynamic simulations of a young low-mass star. We propose a new scenario that can explain observations of the surface abundance of lithium in the Sun and in clusters covering a wide range of ages, from $\sim$ 50 Myr to $\sim$ 4 Gyr. Because it relies on our physical model of convective penetration, this scenario has a limited number of assumptions. It can explain the observed trend between rotation and depletion, based on a single additional assumption, namely that rotation affects the mixing efficiency at the convective boundary. We suggest the existence of a threshold in stellar rotation rate above which rotation strongly prevents the vertical penetration of plumes and below which rotation has small effects. In addition to providing a possible explanation for the long standing problem of lithium depletion in pre-main sequence and main sequence stars, the strength of our scenario is that its basic assumptions can be tested by future hydrodynamic simulations. △ Less

Submitted 29 July, 2017; originally announced July 2017.

Comments: 7 pages, 3 figures, Accepted for publication in ApJ Letters