153 research outputs found
Impact of asteroseismology on improving stellar ages determination
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.Comment: 10 pages, 2 figures, to be published in the proceedings of the
conference "New advances in stellar physics: from microscopic to macroscopic
processes" held at Roscoff, France. EAS Publications Series, 63 (2013)
123-13
Ages of Exoplanet Host-Stars from Asteroseismology : HD 17156, a Case Study
The characterization of the growing number of newly discovered exoplanets
---nature, internal structure, formation and evolution--- strongly relies on
the properties of their host-star, i.e. its mass, radius and age. These latter
can be inferred from stellar evolution models constrained by the observed
global parameters of the host-star --- effective temperature, photospheric
chemical composition, surface gravity and/or luminosity--- and by its mean
density inferred from the transit analysis. Additional constraints for the
models can be provided by asteroseismic observations of the host-star. The
precision and accuracy on the age, mass and radius not only depend on the
quality and number of available observations of the host-star but also on our
ability to model it properly. Stellar models are still based on a number of
approximations, they rely on physical inputs and data that can be uncertain and
do not treat correctly all the physical processes that can be at work inside a
star. We focus here on the determination of the age of HD 17156, an oscillating
star hosting an exoplanet. We examine the dispersion of the age values obtained
by different methods ---empirical or model-dependent--- and the different
sources of errors ---observational or theoretical--- that intervene in the age
determination based on stellar models.Comment: 8 pages, 5 figures, to appear in the ASP proceedings of "The 61st
Fujihara seminar: Progress in solar/stellar physics with helio- and
asteroseismology", 13th-17th March 2011, Hakone, Japan. Ed: Hiromoto
Shibahash
Reference grids of stellar models and oscillation frequencies: Data from the CESAM stellar evolution code and ADIPLS oscillation programme
We present grids of stellar models and their associated oscillation
frequencies that have been used by the CoRoT Seismology Working Group during
the scientific preparation of the CoRoT mission. The stellar models have been
calculated with the CESAM stellar internal structure and evolution code while
the oscillation frequencies have been obtained from the CESAM models by means
of the ADIPLS adiabatic oscillation programme. The grids cover a range of
masses, chemical compositions and evolutionary stages corresponding to those of
the CoRoT primary targets. The stellar models and oscillation frequencies are
available on line through the Evolution and Seismic Tools Activity (ESTA) web
site.Comment: 5 pages, 3 figures accepted for publication in ApSS (CoRoT/ESTA
special volume
Asteroseismology for "\`{a} la carte" stellar age-dating and weighing: Age and mass of the CoRoT exoplanet host HD 52265
In the context of CoRoT, Kepler, Gaia, TESS, and PLATO, precise and accurate
stellar ages, masses and radii are of paramount importance. They are crucial to
constrain scenarii of planetary formation and evolution.We aim at quantifying
how detailed stellar modeling improves the accuracy and precision on age and
mass of individual stars. We adopt a multifaceted approach where we examine how
the number of observational constraints as well as the uncertainties on
observations and on model input physics impact the age-dating and weighing. We
modelled the exoplanet host-star HD52265, a MS, solar-like oscillator observed
by CoRoT. We considered different sets of observational constraints (HR data,
metallicity, seismic constraints). For each case, we determined the age, mass,
and properties of HD52265 inferred from models, and quantified the impact of
the models inputs. Our seismic analysis provides an age A=2.10-2.54 Gyr, a mass
M=1.14-1.32 Msun, and a radius R=1.30-1.34 Rsun, which corresponds to
uncertainties of 10, 7, and 1.5% respectively. Our seismic study provides
constraints on surface convection, through the mixing-length found to be 12-15%
smaller than the solar one. Because of helium-mass degeneracy, the initial He
abundance is determined modulo the mass. The seismic mass of the exoplanet is
found to be Mp sin i=1.17-1.26 MJup, much more precise than what can be derived
by HR diagram inversion. We demonstrate that asteroseismology allows to improve
the age accuracy compared to other methods. We emphasize that the knowledge of
the mean properties of oscillations -as the large frequency separation- is not
enough for deriving accurate ages. We need precise individual frequencies to
narrow the age scatter due to model uncertainties. This strengthen the case for
precise classical stellar parameters and frequencies as will be obtained by
Gaia and PLATO.Comment: 23 pages, 9 figures, Accepted for publication in Astronomy &
Astrophysics Corrected by the language editor, Table link to CD
Stellar convective cores as dark matter probes
The recent detection of a convective core in a main-sequence solar-type star
is used here to test particular models of dark matter (DM) particles, those
with masses and scattering cross sections in the range of interest for the DM
interpretation of the positive results in several DM direct detection
experiments. If DM particles do not effectively self-annihilate after
accumulating inside low-mass stars (e.g. in the asymmetric DM scenario) their
conduction provides an efficient mechanism of energy transport in the stellar
core. For main-sequence stars with masses between 1.1 and 1.3 Msun, this
mechanism may lead to the suppression of the inner convective region expected
to be present in standard stellar evolution theory. The asteroseismic analysis
of the acoustic oscillations of a star can prove the presence/absence of such a
convective core, as it was demonstrated for the first time with the Kepler
field main-sequence solar-like pulsator, KIC 2009505. Studying this star we
found that the asymmetric DM interpretation of the results in the CoGeNT
experiment is incompatible with the confirmed presence of a small convective
core in KIC 2009505.Comment: to appear on Physical Review
CESAM: a free code for stellar evolution calculations
The Cesam code is a consistent set of programs and routines which perform
calculations of 1D quasi-hydrostatic stellar evolution including microscopic
diffusion of chemical species and diffusion of angular momentum. The solution
of the quasi-static equilibrium is performed by a collocation method based on
piecewise polynomials approximations projected on a B-spline basis; that allows
stable and robust calculations, and the exact restitution of the solution, not
only at grid points, even for the discontinuous variables. Other advantages are
the monitoring by only one parameter of the accuracy and its improvement by
super-convergence. An automatic mesh refinement has been designed for adjusting
the localisations of grid points according to the changes of unknowns. For
standard models, the evolution of the chemical composition is solved by stiffly
stable schemes of orders up to four; in the convection zones mixing and
evolution of chemical are simultaneous. The solution of the diffusion equation
employs the Galerkin finite elements scheme; the mixing of chemicals is then
performed by a strong turbulent diffusion. A precise restoration of the
atmosphere is allowed for.Comment: 13 pages, 1 figure, accepted for publication in Astrophysics & Space
Science, ESTA/CoRoT Volum
Comparative seismology of pre- and main sequence stars in the instability strip
Pulsational properties of 1.8 M stellar models covering the latest
stages of contraction toward the main sequence up to early hydrogen burning
phases are investigated by means of linear nonadiabatic analyses. Results
confirm that pre-main sequence stars (pms) which cross the classical
instability strip on their way toward the main sequence are pulsationally
unstable with respect to the classical opacity mechanisms. For both pms and
main sequence types of models in the lower part of the instability strip, the
unstable frequency range is found to be roughly the same. Some non-radial
unstable modes are very sensitive to the deep internal structure of the star.
It is shown that discrimination between pms and main sequence stages is
possible using differences in their oscillation frequency distributions in the
low frequency range.Comment: 8 pages, 9 figures, accepted for publication in A&
Ledoux's convection criterion in evolution and asteroseismology of massive stars
Saio et al. (2006) have shown that the presence of an intermediate convective
zone (ICZ) in post-main sequence models could prevent the propagation of
g-modes in the radiative interior and hence avoid the corresponding radiative
damping. The development of such a convective region highly depends on the
structure of the star in the mu-gradient region surrounding the convective core
during the main sequence phase. In particular,the development of this ICZ
depends on physical processes such as mass loss, overshooting (Chiosi & Maeder
1986, Chiosi et al. 1992, see also Godart et al., these proceedings) and
convective instability criterion (Schwarzschild's or Ledoux's criteria). In
this paper we study the consequences of adopting the Ledoux's criterion on the
evolution of the convective regions in massive stars (15 and 20 Msun), and on
the pulsation spectrum of these new B-type variables (also called SPBsg).Comment: Contribution to the Proceedings of the 38th LIAC/HELAS-ESTA/BAG, 2008
Accepted for publication in CoAs
Stellar ages from asteroseismology
Asteroseismology provides powerful means to probe stellar interiors. The
oscillations frequencies are closely related to stellar interior properties via
the density and sound speed profiles. Since these are tightly linked with the
mass and evolutionary state, we can expect to determine the age and mass of a
star from the comparison of its oscillation spectrum with predictions of
stellar models. Such a comparison suffers both from the problems we face when
modeling a particular star (as the uncertainties on global parameters and
chemical composition) and from our misunderstanding of processes at work in
stellar interiors (as the transport processes that may lead to core mixing and
affect the model ages). For stars where observations have provided precise and
numerous oscillation frequencies together with accurate global parameters and
additional information (as the radius or the mass if the star is in a binary
system, the interferometric radius or the mean density if the star is an
exoplanet host), we can also expect to better constrain the physical
description of the stellar structure and to get a more reliable age estimation.
After a survey of stellar pulsations, we present some seismic diagnostics that
can be used to infer the age of a star as well as their limitations. We then
illustrate the ability of asteroseismology to scrutinize stellar interiors on
the basis of a few exemples. In the years to come, extended very precise
asteroseismic observations are expected, in photometry or in spectroscopy, from
ground-based (HARPS, CORALIE, ELODIE, UVES, UCLES, SIAMOIS, SONG) or spatial
devices (MOST, CoRoT, WIRE, Kepler, PLATO). This will considerably enlarge the
sample of stars eligible to asteroseismic age determination and should allow to
estimate the age of individual stars with a 10-20% accuracy.Comment: 10 pages, 15 figures, Proc. of the IAU Symp. 258 "The Ages of Stars",
Baltimore USA 13-17 Oct 2008, eds D. Soderblom et al., CUP in pres
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