Calibrating angular momentum transport in intermediate-mass stars from gravity-mode asteroseismology

The physical mechanisms driving the transport of angular momentum in stars are not fully understood, as current models cannot explain the observed stellar rotation profiles across all stages of evolution. By making use of pulsating F-type dwarfs, this work aims at (i) observationally calibrating the efficiency of angular momentum transport, assuming a constant uniform viscosity, and (ii) testing how well state-of-the-art rotating stellar models with angular momentum (AM) transport by rotationally-induced processes can explain observed rotation profiles. In both cases, the aim is to simultaneously reproduce the measured near-core rotation and core-to-surface rotation ratio. Asteroseismic modelling is applied to a sample of seven slowly rotating pulsators, to derive (core) masses and ages from their gravity-mode oscillations. This work focuses on the main sequence, using models that start with an initial uniform rotation frequency at the start of core-hydrogen burning that is a free parameter. Two treatments of AM transport are considered: (i) a constant uniform viscosity, and (ii) rotationally-induced processes. Next, the initial rotation frequency of each star is derived from the observed present-day near-core rotation frequency for both treatments. To explain the near-core rotation rate at the inferred age, initial rotation frequencies at the zero-age main sequence need to be below 10 percent of the initial critical break-up frequency. A diffusive approximation of angular momentum transport can in general explain the observed rotation profiles of the six slowly-rotating F-type dwarfs, for average values of the viscosity between 2x10^5 and 5x10^7 cm^2/s or when the viscosity is computed from rotationally-induced mechanisms. Yet, for three stars in the sample, the core-to-surface rotation fraction from rotationally-induced mechanisms is predicted to be higher than observed.Comment: Accepted for publication in Astronomy & Astrophysics, 13 page

The first two-dimensional stellar structure and evolution models of rotating stars

Rotation is a key ingredient in the theory of stellar structure and evolution. Until now, stellar evolution codes operate in a 1-D framework for which the validity domain in regards to the rotation rate is not well understood. This letter aims at presenting the first results of self-consistent stellar models in two spatial dimensions that compute the time evolution of a star and its rotation rate along the main sequence together with a comparison to observations. We make use of an extended version of the ESTER code that solves the stellar structure of a rotating star in two dimensions with time evolution, including chemical evolution, and an implementation of rotational mixing. We have computed evolution tracks for a 12Msun model, once for an initial rotation rate equal to 15% of the critical frequency, and once for 50%. We first show that our model initially rotating at 15% of the critical frequency is able to reproduce all the observations of the $\beta$ Cephei star HD 192575 recently studied by Burssens et al. with asteroseismology. Beyond the classical surface parameters like effective temperature or luminosity, our model also reproduces the core mass along with the rotation rate of the core and envelope at the estimated age of the star. This particular model also shows that the meridional circulation has a negligible influence on the transport of chemical elements, like nitrogen, for which the abundance may be increased at the stellar surface. Furthermore, it shows that in the late main sequence, nuclear evolution is faster than the relaxation time needed to reach a steady state of the star angular momentum distribution. We have demonstrated that we have successfully taken the new step towards 2-D evolutionary modelling of rotating stars. It opens new perspectives on the understanding of the dynamics of fast rotating stars and on the way rotation impacts stellar evolution.Comment: Accepted for publication in Astronomy & Astrophysics Letters, 9 page

Constraining stellar evolution theory with asteroseismology of γ\gamma Doradus stars using deep learning

The efficiency of the transport of angular momentum and chemical elements inside intermediate-mass stars lacks proper calibration, thereby introducing uncertainties on a star's evolutionary pathway. Improvements require better estimation of stellar masses, evolutionary stages, and internal mixing properties. We aim to develop a neural network approach for asteroseismic modelling and test its capacity to provide stellar masses, ages, and overshooting parameter for a sample of 37 $\gamma$ Doradus stars. Here, our goal is to perform the parameter estimation from modelling of individual periods measured for dipole modes with consecutive radial order. We have trained neural networks to predict theoretical pulsation periods of high-order gravity modes as well as the luminosity, effective temperature, and surface gravity for a given mass, age, overshooting parameter, diffusive envelope mixing, metallicity, and near-core rotation frequency. We have applied our neural networks for Computing Pulsation Periods and Photospheric Observables, C-3PO, to our sample and compute grids of stellar pulsation models for the estimated parameters. We present the near-core rotation rates (from literature) as a function of the inferred stellar age and critical rotation rate. We assess the rotation rates of the sample near the start of the main sequence assuming rigid rotation. Furthermore, we measure the extent of the core overshoot region and find no correlation with mass, age, or rotation. The neural network approach developed in this study allows for the derivation of stellar properties dominant for stellar evolution -- such as mass, age, and extent of core-boundary mixing. It also opens a path for future estimation of mixing profiles throughout the radiative envelope, with the aim to infer those profiles for large samples of $\gamma$ Doradus stars.Comment: 35 pages, 63 figures, accepted for publication in A&

Asteroseismic modeling of gravity modes in slowly rotating A/F stars with radiative levitation

It has been known for several decades that transport of chemical elements is induced by the process of microscopic atomic diffusion. Yet, the effect of atomic diffusion, including radiative levitation, has hardly been studied in the context of gravity mode pulsations of core-hydrogen burning stars. In this paper, we study the difference in the properties of such modes for models with and without atomic diffusion. We perform asteroseismic modeling of two slowly rotating A- and F-type pulsators, KIC11145123 ($f_{\rm rot} \approx0.010~{\rm d}^{-1}$) and KIC9751996 ($f_{\rm rot} \approx0.0696~{\rm d}^{-1}$), respectively, based on the periods of individual gravity modes. For both stars, we find models whose g-mode periods are in very good agreement with the {\it Kepler\/} asteroseismic data, keeping in mind that the theoretical/numerical precision of present-day stellar evolution models is typically about two orders of magnitude lower than the measurement errors. Using the Akaike Information Criterion (AIC) we have made a comparison between our best models with and without diffusion, and found very strong evidence for signatures of atomic diffusion in the pulsations of KIC11145123. In the case of KIC9751996 the models with atomic diffusion are not able to explain the data as well as the models without it. Furthermore, we compare the observed surface abundances with those predicted by the best fitting models. The observed abundances are inconclusive for KIC9751996, while those of KIC11145123 from the literature can better be explained by a model with atomic diffusion.Comment: Accepted for publication in Ap

A calibration point for stellar evolution from massive star asteroseismology

Massive stars are progenitors of supernovae, neutron stars and black holes. During the hydrogen-core burning phase their convective cores are the prime drivers of their evolution, but inferences of core masses are subject to unconstrained boundary mixing processes. Moreover, uncalibrated transport mechanisms can lead to strong envelope mixing and differential radial rotation. Ascertaining the efficiency of the transport mechanisms is challenging because of a lack of observational constraints. Here we deduce the convective core mass and robustly demonstrate non-rigid radial rotation in a supernova progenitor, the $12.0^{+1.5}_{-1.5}$ solar-mass hydrogen-burning star HD 192575, using asteroseismology, TESS photometry, high-resolution spectroscopy, and Gaia astrometry. We infer a convective core mass ($M_{\rm cc} = 2.9^{+0.5}_{-0.8}$ solar masses), and find the core to be rotating between 1.4 and 6.3 times faster than the stellar envelope depending on the location of the rotational shear layer. Our results deliver a robust inferred core mass of a massive star using asteroseismology from space-based photometry. HD 192575 is a unique anchor point for studying interior rotation and mixing processes, and thus also angular momentum transport mechanisms inside massive stars.Comment: 41 pages, 5 figures, 1 table. Version comment: updated erroneous affiliatio

Weighing stars from birth to death : mass determination methods across the HRD

Funding: C.A., J.S.G.M., and M.G.P. received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement No 670519: MAMSIE). N.B. gratefully acknowledge financial support from the Royal Society (University Research Fellowships) and from the European Research Council (ERC-CoG-646928, Multi-Pop).The mass of a star is the most fundamental parameter for its structure, evolution, and final fate. It is particularly important for any kind of stellar archaeology and characterization of exoplanets. There exist a variety of methods in astronomy to estimate or determine it. In this review we present a significant number of such methods, beginning with the most direct and model-independent approach using detached eclipsing binaries. We then move to more indirect and model-dependent methods, such as the quite commonly used isochrone or stellar track fitting. The arrival of quantitative asteroseismology has opened a completely new approach to determine stellar masses and to complement and improve the accuracy of other methods. We include methods for different evolutionary stages, from the pre-main sequence to evolved (super)giants and final remnants. For all methods uncertainties and restrictions will be discussed. We provide lists of altogether more than 200 benchmark stars with relative mass accuracies between [0.3 ,2 ]% for the covered mass range of M ∈[0.1 ,16 ] M⊙ , 75 % of which are stars burning hydrogen in their core and the other 25 % covering all other evolved stages. We close with a recommendation how to combine various methods to arrive at a "mass-ladder" for stars.PostprintPeer reviewe

Weighing stars from birth to death: mass determination methods across the HRD

The mass of a star is the most fundamental parameter for its structure, evolution, and final fate. It is particularly important for any kind of stellar archaeology and characterization of exoplanets. There exists a variety of methods in astronomy to estimate or determine it. In this review we present a significant number of such methods, beginning with the most direct and model-independent approach using detached eclipsing binaries. We then move to more indirect and model-dependent methods, such as the quite commonly used isochrone or stellar track fitting. The arrival of quantitative asteroseismology has opened a completely new approach to determine stellar masses and to complement and improve the accuracy of other methods. We include methods for different evolutionary stages, from the pre-main sequence to evolved (super)giants and final remnants. For all methods uncertainties and restrictions will be discussed. We provide lists of altogether more than 200 benchmark stars with relative mass accuracies between $[0.3,2]\%$ for the covered mass range of M\in [0.1,16]\,\msun, $75\%$ of which are stars burning hydrogen in their core and the other $25\%$ covering all other evolved stages. We close with a recommendation how to combine various methods to arrive at a "mass-ladder" for stars.Comment: Invited review article for The Astronomy and Astrophysics Review. 146 pages, 16 figures, 11 tables. Accepted version by the Journal. It includes summary figure of accuracy/precision of methods for mass ranges and summary table for individual method

Predictions for Gravity-mode Periods and Surface Abundances in Intermediate-mass Dwarfs from Shear Mixing and Radiative Levitation

International audienceThe treatment of chemical mixing in the radiative envelopes of intermediate-mass stars has hardly been calibrated so far. Recent asteroseismic studies demonstrated that a constant diffusion coefficient in the radiative envelope is not able to explain the periods of trapped gravity modes in the oscillation spectra of γ Doradus pulsators. We present a new generation of MESA stellar models with two major improvements. First, we present a new implementation for computing radiative accelerations and Rosseland mean opacities that requires significantly less CPU time. Second, the inclusion of shear mixing based on rotation profiles computed with the 2D stellar structure code ESTER is considered. We show predictions for the mode periods of these models covering stellar masses from 1.4 to 3.0 M ⊙ across the main sequence, computed for different metallicities. The morphology of the chemical mixing profile resulting from shear mixing in combination with atomic diffusion and radiative levitation does allow for mode trapping, while the diffusion coefficient in the outer envelope is large (>106 cm2 s-1). Furthermore, we make predictions for the evolution of surface abundances for which radiative accelerations can be computed. We find that the N/C and C/O abundance ratios correlate with stellar age. We predict that these correlations are observable with precisions ≲ 0.1 dex on these ratios, given that a precise age estimate can be made

A homogeneous spectroscopic analysis of a

Context. Asteroseismic modelling of the internal structure of main-sequence stars born with a convective core has so far been based on homogeneous analyses of space photometric Kepler light curves of four years in duration, to which most often incomplete inhomogeneously-deduced spectroscopic information was added to break degeneracies. Aims. Our goal is twofold: (1) to compose an optimal sample of gravity-mode pulsators observed by the Kepler space telescope for joint asteroseismic and spectroscopic stellar modelling, and (2) to provide spectroscopic parameters for its members, deduced in a homogeneous way. Methods. We assembled HERMES high-resolution optical spectroscopy at the 1.2 m Mercator telescope for 111 dwarfs, whose Kepler light curves allowed for the determination of their near-core rotation rates. Our spectroscopic information offers additional observational input to also model the envelope layers of these non-radially pulsating dwarfs. Results. We determined stellar parameters and surface abundances from atmospheric analysis with spectrum normalisation based on a new machine-learning tool. Our results suggest a systematic overestimation of metallicity ([M/H]) in the literature for the studied F-type dwarfs, presumably due to normalisation limitations caused by the dense line spectrum of these rotating stars. CNO surface abundances were found to be uncorrelated with the rotation properties of the F-type stars. For the B-type stars, we find a hint of deep mixing from C and O abundance ratios; N abundance uncertainties are too great to reveal a correlation of N with the rotation of the stars. Conclusions. Our spectroscopic stellar parameters and abundance determinations allow for the future joint spectroscopic, astrometric (Gaia), and asteroseismic modelling of this legacy sample of gravity-mode pulsators, with the aim of improving our understanding of transport processes in the core-hydrogen burning phase of stellar evolution

Modules for Experiments in Stellar Astrophysics (MESA): Time-dependent Convection, Energy Conservation, Automatic Differentiation, and Infrastructure

peer reviewedWe update the capabilities of the open-knowledge software instrument Modules for Experiments in Stellar Astrophysics (MESA). The new auto_diff module implements automatic differentiation in MESA, an enabling capability that alleviates the need for hard-coded analytic expressions or finite-difference approximations. We significantly enhance the treatment of the growth and decay of convection in MESA with a new model for time-dependent convection, which is particularly important during late-stage nuclear burning in massive stars and electron-degenerate ignition events. We strengthen MESA’s implementation of the equation of state, and we quantify continued improvements to energy accounting and solver accuracy through a discussion of different energy equation features and enhancements. To improve the modeling of stars in MESA, we describe key updates to the treatment of stellar atmospheres, molecular opacities, Compton opacities, conductive opacities, element diffusion coefficients, and nuclear reaction rates. We introduce treatments of starspots, an important consideration for low-mass stars, and modifications for superadiabatic convection in radiation-dominated regions. We describe new approaches for increasing the efficiency of calculating monochromatic opacities and radiative levitation, and for increasing the efficiency of evolving the late stages of massive stars with a new operator-split nuclear burning mode. We close by discussing major updates to MESA’s software infrastructure that enhance source code development and community engagement