350 research outputs found
Probing the parameter space of HD 49933: a comparison between global and local methods
We present two independent methods for studying the global stellar parameter
space (mass M, age, initial chemical composition X_0, Z_0) of HD 49933 with
seismic data. Using a local minimization and an MCMC algorithm, we obtain
consistent results for the determination of the stellar properties: M = 1.1 -
1.2 M_solar, Age ~ 3.0 Gyr, Z_0 ~ 0.008. A description of the error ellipses
can be defined using Singular Value Decomposition techniques, and this is
validated by comparing the errors with those from the MCMC method.Comment: to be published in JPC
Uncertainties and biases in modelling 16 Cyg A and B
In this study I assess how existing data for the solar analogues 16 Cyg A and
B, in particular the asteroseismic measurements obtained from \emph{Kepler},
constrain theoretical stellar models. The goal is two-fold: first to use these
stars as benchmarks to discuss which precisions can realistically be expected
on the inferred stellar quantities; and second to determine how well
'non-standard' prescriptions, such as microscopic diffusion and overshoot, are
constrained. I used a Bayesian statistical model to infer the values of the
stellar parameters of 16 Cyg A and B. I sampled the posterior density of the
stellar parameters via a Markov chain Monte Carlo (MCMC) algorithm, tested
different physical prescription, and examined the impact of using different
seismic diagnostics. General good agreement is found with several recent
modelling studies on these stars, even though some discrepancies subsist
regarding the precise estimates of the uncertainties on the parameters. An age
of Gyr is estimated for the binary system. The inferred masses,
M for Cyg A and M for Cyg B,
are shown to be stable with respect to changes in the physical prescriptions
considered for the modelling. For both stars, microscopic diffusion has a
significant effect on the estimates of the initial metallicity. Overshoot is
confined to very small regions below the convective zone. I show that a proper
treatment of the seismic constraints is necessary to avoid biases in the
estimate of the mass.Comment: 13 pages, 7 figures, to appear in A &
Asteroseismic Stellar Modelling with AIMS
The goal of AIMS (Asteroseismic Inference on a Massive Scale) is to estimate
stellar parameters and credible intervals/error bars in a Bayesian manner from
a set of asteroseismic frequency data and so-called classical constraints. To
achieve reliable parameter estimates and computational efficiency, it searches
through a grid of pre-computed models using an MCMC algorithm -- interpolation
within the grid of models is performed by first tessellating the grid using a
Delaunay triangulation and then doing a linear barycentric interpolation on
matching simplexes. Inputs for the modelling consist of individual frequencies
from peak-bagging, which can be complemented with classical spectroscopic
constraints. AIMS is mostly written in Python with a modular structure to
facilitate contributions from the community. Only a few computationally
intensive parts have been rewritten in Fortran in order to speed up
calculations.Comment: 11 pages, 4 figures. Tutorial presented at the IVth Azores
International Advanced School in Space Sciences on "Asteroseismology and
Exoplanets: Listening to the Stars and Searching for New Worlds"
(arXiv:1709.00645), which took place in Horta, Azores Islands, Portugal in
July 201
Asteroseismology of the planet-hosting star mu Arae. II. Seismic analysis
As most exoplanets host stars, HD 160691 (alias mu Ara) presents a
metallicity excess in its spectrum compared to stars without detected planets.
This excess may be primordial, in which case the star would be completely
overmetallic, or it may be due to accretion in the early phases of planetary
formation, in which case it would be overmetallic only in its outer layers. As
discussed in a previous paper (Bazot and Vauclair 2004), seismology can help
choosing between the two scenarios. This star was observed during eight nights
with the spectrograph HARPS at La Silla Observatory. Forty three p-modes have
been identified (Bouchy et al. 2005). In the present paper, we discuss the
modelisation of this star. We computed stellar models iterated to present the
same observable parameters (luminosity, effective temperature, outer chemical
composition) while the internal structure was different according to the two
extreme assumptions : original overmetallicity or accretion. We show that in
any case the seismic constraints lead to models in complete agreement with the
external parameters deduced from spectroscopy and from the Hipparcos parallax
(L and Teff). We discuss the tests which may lead to a choice between the two
typical scenarios. We show that the ``small separation'' seem to give a better
fit for the accretion case than for the overmetallic case, but in spite of the
very good data the uncertainties are still too large to conclude. We discuss
the observations which would be needed to go further and solve this question.Comment: 16 pages, 8 figures, accepted in A&
The CoRoT primary target HD 52265: models and seismic tests
HD 52265 is the only known exoplanet-host star selected as a main target for
the seismology programme of the CoRoT satellite. As such, it will be observed
continuously during five months, which is of particular interest in the
framework of planetary systems studies. This star was misclassified as a giant
in the Bright Star Catalog, while it is more probably on the main-sequence or
at the beginning of the subgiant branch. We performed an extensive analysis of
this star, showing how asteroseismology may lead to a precise determination of
its external parameters and internal structure. We first reviewed the
observational constraints on the metallicity, the gravity and the effective
temperature derived from the spectroscopic observations of HD 52265. We also
derived its luminosity using the Hipparcos parallax. We computed the
evolutionary tracks for models of various metallicities which cross the
relevant observational error boxes in the gravity-effective temperature plane.
We selected eight different stellar models which satisfy the observational
constraints, computed their p-modes frequencies and analysed specific seismic
tests. The possible models for HD 52265, which satisfy the constraints derived
from the spectroscopic observations, are different in both their external and
internal parameters. They lie either on the main sequence or at the beginning
of the subgiant branch. The differences in the models lead to quite different
properties of their oscillation frequencies. We give evidences of an
interesting specific behaviour of these frequencies in case of helium-rich
cores: the ``small separations'' may become negative and give constraints on
the size of the core. We expect that the observations of this star by the CoRoT
satellite wi ll allow choosing between these possible models.Comment: 11 pages, 7 figures, to be published in Astronomy and Astrophysic
Asteroseismology of exoplanets host stars: the special case of Horologii (HD17051)
{This paper presents detailed analysis and modelisation of the star HD17051
(alias Hor), which appears as a specially interesting case among
exoplanet host stars. As most of these stars, Hor presents a
metallicity excess which has been measured by various observers who give
different results, ranging from [Fe/H] = 0.11 to 0.26, associated with
different atmospheric parameters. Meanwhile the luminosity of the star may be
determined owing to Hipparcos parallax. Although in the southern hemisphere,
this star belongs to the Hyades stream and its external parameters show that it
could even be one of the Hyades stars ejected during cluster formation. The aim
of this work was to gather and analyse our present knowledge on this star and
to prepare seismic tests for future observations with the HARPS spectrometer
(planned for November 2006).} {We have computed evolutionary tracks with
various metallicities, in the two frameworks of primordial overmetallicity and
accretion. We have concentrated on models inside the error boxes given by the
various observers in the log g - log T diagram. We then computed the
adiabatic oscillation frequencies of these models to prepare future
observations.} {The detailed analysis of Hor presented in this paper
already allowed to constrain its external parameters, mass and age. Some values
given in the literature could be rejected as inconsistent with the overall
analysis. We found that a model computed with the Hyades parameters (age,
metallicity) was clearly acceptable, but other ones were possible too. We are
confident that observations with HARPS will allow for a clear conclusion about
this star and that it will bring important new light on the physics of
exoplanet host stars.}Comment: to be published in Astronomy and Astrophysic
The solar, exoplanet and cosmological lithium problems
We review three Li problems. First, the Li problem in the Sun, for which some
previous studies have argued that it may be Li-poor compared to other Suns.
Second, we discuss the Li problem in planet hosting stars, which are claimed to
be Li-poor when compared to field stars. Third, we discuss the cosmological Li
problem, i.e. the discrepancy between the Li abundance in metal-poor stars
(Spite plateau stars) and the predictions from standard Big Bang
Nucleosynthesis. In all three cases we find that the "problems" are naturally
explained by non-standard mixing in stars.Comment: Astrophysics and Space Science, in press. New version has one
reference correcte
Asteroseismic detection of latitudinal differential rotation in 13 Sun-like stars
The differentially rotating outer layers of stars are thought to play a role
in driving their magnetic activity, but the underlying mechanisms that generate
and sustain differential rotation are poorly understood. We report the
measurement of latitudinal differential rotation in the convection zones of 40
Sun-like stars using asteroseismology. For the most significant detections, the
stars' equators rotate approximately twice as fast as their mid-latitudes. The
latitudinal shear inferred from asteroseismology is much larger than
predictions from numerical simulations.Comment: 45 pages, 11 figures, 4 tables, published in Scienc
Asteroseismic signatures of helium gradients in late F-type stars
Element diffusion is expected to occur in all kinds of stars : according to
the relative effect of gravitation and radiative acceleration, they can fall or
be pushed up in the atmospheres. Helium sinks in all cases, thereby creating a
gradient at the bottom of the convective zones. This can have important
consequences for the sound velocity, as has been proved in the sun with
helioseismology.
We investigate signatures of helium diffusion in late F-type stars by
asteroseismology.
Stellar models were computed with different physical inputs (with or without
element diffusion) and iterated in order to fit close-by evolutionary tracks
for each mass. The theoretical oscillation frequencies were computed and
compared for pairs of models along the tracks. Various asteroseismic tests
(large separations, small separations, second differences) were used and
studied for the comparisons.
The results show that element diffusion leads to changes in the frequencies
for masses larger than 1.2 Msun. In particular the helium gradient below the
convective zone should be detectable through the second differences.Comment: 8 pages, 11 figures, 2 tables Accepted for publication in Astronomy
and Astrophysics. The official date of acceptance is 03/05/200
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