11 research outputs found
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 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
New seismic analysis of the exoplanet-host star Mu Arae
We present detailed modelling of the exoplanet-host star Mu Arae, using a new
method for the asteroseismic analysis, and taking into account the new value
recently derived for the Hipparcos parallax. The aim is to obtain precise
parameters for this star and its internal structure, including constraints on
the core overshooting. We computed new stellar models in a wider range than
Bazot et al. (2005), with various chemical compositions ([Fe/H] and Y), with or
without overshooting at the edge of the core. We computed their adiabatic
oscillation frequencies and compared them to the seismic observations. For each
set of chemical parameters, we kept the model which represented the best fit to
the echelle diagram. Then, by comparing the effective temperatures, gravities
and luminosities of these models with the spectroscopic error boxes, we were
able to derive precise parameters for this star. First we find that all the
models which correctly fit the echelle diagram have the same mass and radius,
with an uncertainty of the order of one percent. Second, the final comparison
with spectroscopic observations leads to the conclusion that besides its high
metallicity, Mu Arae has a high helium abundance of the order of Y=0.3. Knowing
this allows finding precise values for all the other parameters, mass, radius
and age.Comment: Accepted for publication in A&
The exoplanet-host star
Aims. We show that the exoplanet-host star iota Horologii, alias HD 17051, which belongs to the so-called Hyades stream, was formed within the primordial Hyades stellar cluster and has evaporated towards its present location, 40 pc away.
Methods. This result has been obtained unambiguously by studying the acoustic oscillations of this star, using the HARPS spectrometer in La Silla Observatory (ESO, Chili).
Results. Besides the fact that ι Hor belongs to the Hyades stream, we give evidence that it has the same metallicity, helium abundance, and age as the other stars of the Hyades cluster. They were formed together, at the same time, in the same primordial cloud.
Conclusions. This result has strong implications for theories of stellar formation. It also indicates that the observed overmetallicity of this exoplanet-host star, about twice that of the Sun, is original and not caused by planet accretion during the formation of the planetary system