In the decade to come major improvements are expected in the field of stellar physics coming from all sides: theory, laboratory and numerical experiments, and observation. We illustrate some aspects ...of the expected progress on the basis of a few exemples concerning the input physics and parameters of the stellar models and the asteroseismic diagnostics.
The analysis of the available TESS light curves of $\alpha$ Sex (HD 87887)
reveals low-frequency pulsations with a period of about 9.1 hours in this
spectroscopic A0 III standard star. The IUE ...observations in December 1992
reveal large flux variations both in the far UV and in the mid UV which are
accompanied by variations of the brightness in the V band recorded by the Fine
Error Sensor on board IUE. The ultraviolet variability could be due to an
eclipse by an hitherto undetected companion of smaller radius, possibly 2.5
R$_{\odot}$ but this needs confirmation by further monitoring possibly with
TESS. An abundance determination yields solar abundances for most elements.
Only carbon and strontium are underabundant and titanium, vanadium and barium
mildly overabundant. Identification is provided for most of the lines absorbing
more than 2% in the optical spectrum of $\alpha$ Sex. Stellar evolution
modeling shows that $\alpha$ Sex is near the terminal-age main sequence, and
its mass, radius and age are estimated to be $M = 2.57 \pm 0.32$ M$_{\odot}$,
$R = 3.07 \pm 0.90$ R$_{\odot}$, $A = 385 \pm 77$ Myr, respectively.
A comparison between the oscillation frequencies of six multi-periodic δ Scuti stars of the Pleiades cluster and the eigenfrequencies of rotating stellar models that match the corresponding stellar ...parameters has been carried out. The assumption that all the stars considered have some common parameters, such as metallicity, distance or age, is imposed as a constraint. As a result, we have a best fit solution associated with a cluster metallicity of $\rm Fe/H\simeq 0.067$, an age between $70 \times 10^{6}$ and $100 \times 10^{6}\,$yr and a distance modulus of $m_{V}-M_{V}=5.60$–5.70 mag. All the stars were found to oscillate mainly in non-radial, low degree, low order p modes. Estimates of mass and rotation rates for each star are also obtained.
We compute the rates P at which acoustic energy is injected into the solar radial p modes for several solar models. The solar models are computed with two different local treatments of convection: ...the classical mixing-length theory (MLT) and the formulation by Canuto et al. (1996, ApJ, 473, 550, CGM). Among the models investigated here, our best models reproduce both (i) the solar radius and the solar luminosity at solar age and (ii) the observed Balmer line profiles. For the MLT treatment, the rates P do significantly depend on the properties of the atmosphere, whereas for the CGM treatment, the dependence of P on the properties of the atmosphere is found to be smaller than the error bars attached to the seismic measurements. The excitation rates P for modes associated with the MLT models are significantly underestimated compared with the solar seismic constraints. The CGM models yield values for P closer to the seismic data than do the MLT models. We conclude that the solar p-mode excitation rates provide valuable constraints and, according to the present investigation, clearly favor the CGM treatment with respect to the MLT, although neither of them yields values of P as close to the observations as recently found for 3D numerical simulations.
High precision photometry as performed by the CoRoT and Kepler satellites on-board instruments has allowed to detect stellar oscillations over the whole HR diagram. Oscillation frequencies are ...closely related to stellar interior properties via the density and sound speed profiles, themselves tightly linked with the mass and evolutionary state of stars. Seismic diagnostics performed on stellar internal structure models allow to infer the age and mass of oscillating stars. The accuracy and precision of the age determination depend both on the goodness of the observational parameters (seismic and classical) and on our ability to model a given star properly. They therefore suffer from any misunderstanding of the physical processes at work inside stars (as microscopic physics, transport processes...). In this paper, we recall some seismic diagnostics of stellar age and we illustrate their efficiency in age-dating the CoRoT target HD 52265.
The understanding and modelling of the structure and evolution of stars is based on statistical physics as well as on hydrodynamics. Today, a precise identification and proper description of the ...physical processes at work in stellar interiors are still lacking (one key point being that of transport processes) while comparison of real stars to model predictions, which implies conversions from the theoretical space to the observational one, suffers from uncertainties in model atmospheres. This results in uncertainties on the prediction of stellar properties needed for galactic studies or cosmology (as stellar ages and masses). In the next decade, progress is expected from the theoretical, experimental and observational sides. I illustrate some of the problems we are facing when modelling stars and possible ways toward their solutions. I discuss how future observational ground-based or spatial programs (in particular those dedicated to micro-arc-second astrometry, asteroseismology and interferometry) will provide precise determinations of the stellar parameters and contribute to a better knowledge of stellar interiors and atmospheres in a wide range of stellar masses, chemical composition and evolution stages.
In order to make asteroseismology a powerful tool to explore stellar interiors, different numerical codes should give the same oscillation frequencies for the same input physics. Any differences ...found when comparing the numerical values of the eigenfrequencies will be an important piece of information regarding the numerical structure of the code. The ESTA group was created to analyze the non-physical sources of these differences. The work presented in this report is a part of Task 2 of the ESTA group. Basically the work is devoted to test, compare and, if needed, optimize the seismic codes used to calculate the eigenfrequencies to be finally compared with observations. The first step in this comparison is presented here. The oscillation codes of nine research groups in the field have been used in this study. The same physics has been imposed for all the codes in order to isolate the non-physical dependence of any possible difference. Two equilibrium models with different grids, 2172 and 4042 mesh points, have been used, and the latter model includes an explicit modelling of semiconvection just outside the convective core. Comparing the results for these two models illustrates the effect of the number of mesh points and their distribution in particularly critical parts of the model, such as the steep composition gradient outside the convective core. A comprehensive study of the frequency differences found for the different codes is given as well. These differences are mainly due to the use of different numerical integration schemes. The number of mesh points and their distribution are crucial for interpreting the results. The use of a second-order integration scheme plus a Richardson extrapolation provides similar results to a fourth-order integration scheme. The proper numerical description of the Brunt-Väisälä frequency in the equilibrium model is also critical for some modes. This influence depends on the set of the eigenfunctions used for the solution of the differential equations. An unexpected result of this study is the high sensitivity of the frequency differences to the inconsistent use of values of the gravitational constant (
G
) in the oscillation codes, within the range of the experimentally determined ones, which differ from the value used to compute the equilibrium model. This effect can provide differences for a given equilibrium model substantially larger than those resulting from the use of different codes or numerical techniques; the actual differences between the values of
G
used by the different codes account for much of the frequency differences found here.
Context.The calibration of binary systems with accurately known masses and/or radii provides powerful tools to test stellar structure and evolution theory and to determine the age and helium content ...of stars. We study the eclipsing double-lined spectroscopic binary system RS Cha, for which we have accurate observations of the parameters of both stars (masses, radii, luminosities, effective temperatures and metallicity). Aims.We have calculated several sets of stellar models for the components of the RS Cha system, with the aim of reproducing simultaneously the available observational constraints and to estimate the age and initial helium abundance of the system. Methods.Using the CESAM stellar evolution code, we model both components starting from the initial mass and metallicity and adjusting the input parameters and physics in order to satisfy the observational constraints. Results.We find that the observations cannot be reproduced if we assume that the abundance ratios are solar but they are satisfied if carbon and nitrogen are depleted in the RS Cha system with respect to the Sun. This is in accordance with the abundances observed in other young stars. The RS Cha system is in an evolutionary stage at the end of the PMS phase where models are not strongly sensitive to various physical uncertainties. However we show that the oscillations of these two stars, which have been detected, would be able to discriminate between different options in the physical description of this evolutionary phase.
Two problems concerning the faintest Small Magellanic Cloud (SMC) Cepheids are addressed. On the one hand evolutionary tracks fail to cross the Cepheid Instability Strip for the highest magnitudes ...(i.e. I-mag $\sim 17$) where Cepheids are observed; mass–luminosity relations (ML) obtained from evolutionary tracks disagree with mass–luminosity relations derived from observations. We find that the above failures concern models built with standard input physics as well as with non-standard ones. The present work suggests that towards highest magnitudes, Cepheids stars undergo a selection effect caused by evolution: only the most metal poor stars cross the Instability Strip during the “blue loop” phase and are therefore the only ones that can be observed at low luminosity. This solution enables us to reproduce the shape of the lower part of the Instability Strip and improves the agreement between observed and theoretical ML-relations. Some issues are discussed, among them Beat Cepheid results that argue strongly in favor of our hypothesis.
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