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Among the various methods used to age-date stars, methods based on stellar model predictions are widely used, for nearly all kind of stars in large ranges of masses, chemical compositions and evolutionary stages. The precision and accuracy on the age determination depend on both the precision and number of observational constraints, and on our ability to correctly describe the stellar interior and evolution. The imperfect input physics of stellar models as well as the uncertainties on the initial chemical composition of stars are responsible for uncertainties in the age determination. We present an overview of the calculation of stellar models and discuss the impact on age of their numerous inputs.
Accurate and precise stellar ages are best determined for stars which are strongly observationally constrained, that is which are intrinsically oscillating. We review here the seismic diagnostics which are sensitive to stellar ages and provide some illustrating examples of seismically age-dated stars.
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 aim of the MIMAC project is to detect non-baryonic Dark Matter with a directional TPC. The recent Micromegas efforts towards building a large size detector will be described, in particular the characterization measurements of a prototype detector of 10 × 10 cm2 with a 2 dimensional readout plane. Track reconstruction with alpha particles will be shown.
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
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.
We present recent work undertaken by the Evolution and Seismic Tools Activity (ESTA) team of the CoRoT Seismology Working Group. The new ESTA-Task 3 aims at testing, comparing and optimising stellar evolution codes which include microscopic diffusion of the chemical elements resulting from pressure, temperature and concentration gradients. The results already obtained are globally satisfactory, but some differences between the different numerical tools appear that require further investigations.
With the first light of COROT, the preparation of KEPLER and the future helioseismology spatial projects such as GOLF-NG, a coherent picture of the evolution of rotating stars from their birth to their death is needed. We describe here the modelling of the macroscopic transport of angular momentum and matter in stellar interiors that we have undertaken to reach this goal. First, we recall in detail the dynamical processes that are driving these mechanisms in rotating stars and the theoretical advances we have achieved. Then, we present our new results of numerical simulations which allow us to follow in 2D the secular hydrodynamics of rotating stars, assuming that anisotropic turbulence enforces a shellular rotation law. Finally, we show how this work is leading to a dynamical vision of the Hertzsprung-Russel diagram with the support of asteroseismology and helioseismology, seismic observables giving constraints on the modelling of the internal transport and mixing processes. In conclusion, we present the different processes that should be studied in the near future to improve our description of stellar radiation zones.
The ESTA activity under the CoRoT project aims at testing the tools for
computing stellar models and oscillation frequencies that will be used
in the analysis of asteroseismic data from CoRoT and other large-scale
upcoming asteroseismic projects.
Here I report results of comparisons between calculations using
the Aarhus code (ASTEC) and two other codes, for models that include
diffusion and settling.
It is found that there are likely deficiencies, requiring further study,
in the ASTEC computation of models including convective cores.
In order to prepare the theoretical interpretation of the oscillation frequencies detected by
CoRoT, comparisons of results from standard stellar models by the ESTA group
have proven to be very useful. The next issue which is briefly addressed here is
“what are the additional physical processes that must be included in stellar
models computed with different evolutionary codes for the next comparison exercises?”
We therefore discuss the impact on oscillation frequencies of several
physical processes which are still poorly understood and/or poorly modelled
but cannot be fully discarded.
Three-dimensional Large Eddy Simulations (LES) of the
convection-radiation transition layer of solar-type stars predict
a different temperature stratification compared to models that use
standard mixing-length theory. When these structural changes are
taken into account theoretical oscillation frequencies are
modified. Here, we compare the changes to the seismic spectra
as predicted by the LES with those arising from diffusion and settling
of helium and heavier elements. It is shown that turbulence effects
in the modelling of the star α Centauri A change the
large frequency separations twice as much as compared to those arising
from gravitational settling, giving rise to a frequency shift
opposite to what would have been expected from gravitational settling
The present work is placed in the framework of the preparation for the interpretation
of the seismic space data from the space mission CoRoT. In this context, rotation have
become one of the most limiting factors when analysing the observed spectra of stars.
A general view on the effects of rotation on the oscillations
is given from the point of view of a linear perturbation analysis.
As well, the latest results about the effect of shellular rotation on adiabatic
oscillation frequencies are addressed. Finally,
the role of rotation on asteroseismic diagnostics is discussed.
In particular, échélle diagrams and analysis of rotation
profile variations are examined.
Gravitational settling and microscopic diffusion affects the main
sequence lifetimes, main sequence turn-off luminosities and
temperatures of metal-poor halo stars. As a result, the inclusion of
diffusion in the stellar evolution models has important consequences
for age determinations of old stars. In contrast, diffusion has
little impact on the post-main sequence evolution of metal-poor stars.
Observations of the heavy element abundances in turn-off and giant
branch stars in the globular cluster NGC 6397 suggest that diffusion
occurs in metal-poor stars, but that the diffusion must be inhibited
by turbulent mixing.