1,129 research outputs found
Comparisons for Esta-Task3: Cles and Cesam
We present the results of comparing three different implementations of the
microscopic diffusion process in the stellar evolution codes CESAM and CLES.
For each of these implementations we computed models of 1.0, 1.2 and 1.3
M. We analyse the differences in their internal structure at three
selected evolutionary stages, as well as the variations of helium abundance and
depth of the stellar convective envelope. The origin of these differences and
their effects on the seismic properties of the models are also considered.Comment: 10 pages, 8 figures, Joint HELAS and CoRoT/ESTA Workshop on
Solar/Stellar Models and Seismic Analysis Tools, Novembre, Porto 2007 To be
published in EAS Publications Serie
Stars in the age of micro-arc-second astrometry
The understanding and modeling 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 the comparison of real stars to model predictions, which
implies conversions from the theoretical space to the observational one,
suffers from uncertainties in model atmospheres. That 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 faced with when modeling stars and the
possible tracks towards 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 compositions and evolution stages.Comment: 7 pages; to appear in the proceedings of "IAU Symposium 248 - A Giant
Step: from Milli- to Micro-arcsecond Astrometry", held in Shanghai, China,
15-19 Oct. 200
Effect of local treatments of convection upon the solar p-mode excitation rates
We compute, for several solar models, the rates P at which the solar radial p
modes are expected to be excited. The solar models are computed with two
different local treatments of convection : the classical mixing-length theory
(MLT hereafter) and Canuto, Goldmann and Mazzitelli(1996, CGM hereafter)'s
formulation. For one set of solar models (EMLT and ECGM models), the atmosphere
is gray and assumes Eddington's approximation. For a second set of models (KMLT
and KCGM models), the atmosphere is built using a T(tau) law which has been
obtained from a Kurucz's model atmosphere computed with the same local
treatment of convection. The mixing-length parameter in the model atmosphere is
chosen so as to provide a good agreement between synthetic and observed Balmer
line profiles, while the mixing-length parameter in the interior model is
calibrated so that the model reproduces the solar radius at solar age. For the
MLT treatment, the rates P do depend significantly on the properties of the
atmosphere. On the other hand, for the CGM treatment, differences in P between
the ECGM and the KCGM models are very small compared to the error bars attached
to the seismic measurements. The excitation rates P for modes from the EMLT
model are significantly under-estimated compared with the solar seismic
constraints. The KMLT model results in intermediate values for P and shows also
an important discontinuity in the temperature gradient and the convective
velocity. On the other hand, the KCGM model and the ECGM model yield values for
P closer to the seismic data than the EMLT and KMLT models. We conclude that
the solar p-mode excitation rates provide valuable constraints and according to
the present investigation cleary favor the CGM treatment with respect to the
MLT.Comment: 4 pages, 3 figures, proceedings of the SOHO14/GONG 2004 workshop
"Helio- and Asteroseismology: Towards a Golden Future" from July 12-16 2004
at New Haven CT (USA
Mode stability in delta Scuti stars: linear analysis versus observations in open clusters
A comparison between linear stability analysis and observations of pulsation
modes in five delta Scuti stars, belonging to the same cluster, is presented.
The study is based on the work by Michel et al. (1999), in which such a
comparison was performed for a representative set of model solutions obtained
independently for each individual star considered. In this paper we revisit the
work by Michel et al. (1999) following, however, a new approach which consists
in the search for a single, complete, and coherent solution for all the
selected stars, in order to constrain and test the assumed physics describing
these objects. To do so, refined descriptions for the effects of rotation on
the determination of the global stellar parameters and on the adiabatic
oscillation frequency computations are used. In addition, a crude attempt is
made to study the role of rotation on the prediction of mode instabilities.The
present results are found to be comparable with those reported by Michel et al.
(1999). Within the temperature range log T_eff = 3.87-3.88 agreement between
observations and model computations of unstable modes is restricted to values
for the mixing-length parameter alpha_nl less or equal to 1.50. This indicates
that for these stars a smaller value for alpha_nl is required than suggested
from a calibrated solar model. We stress the point that the linear stability
analysis used in this work still assumes stellar models without rotation and
that further developments are required for a proper description of the
interaction between rotation and pulsation dynamics.Comment: 8 pages, 4 figures, 3 tables. (MNRAS, in press
Asteroseismology of delta Scuti stars in open clusters: Praesepe
The present paper provides a general overview of the asteroseismic potential
of delta Scuti stars in clusters, in particular focusing on convection
diagnostics. We give a summarise of the last results obtained by the authors
for the Praesepe cluster of which five delta Scuti stars are analysed. In that
work, linear analysis is confronted with observations, using refined
descriptions for the effects of rotation on the determination of the global
stellar parameters and on the adiabatic oscillation frequency computations. A
single, complete, and coherent solution for all the selected stars is found,
which lead the authors to find important restrictions to the convection
description for a certain range of effective temperatures. Furthermore, the
method used allowed to give an estimate of the global parameters of the
selected stars and constrain the cluster.Comment: 6 pages, 1 figure. Accepted for publication in Communications in
Asteroseismolog
Influence of local treatments of convection upon solar p mode excitation rates
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 hereafter) and Canuto et al (1996)'s formulation (CGM hereafter). Among
the models investigated here, our best models reproduce both the solar radius
and the solar luminosity at solar age and the observed Balmer line profiles.
For the MLT treatment, the rates P do depend significantly on the properties of
the atmosphere whereas for the CGM's treatment the dependence of P on the
properties of the atmosphere is found 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
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.Comment: 11 pages, 7 figures, accepted for publication in Astronomy &
Astrophysic
Seismic diagnostics for transport of angular momentum in stars 2. Interpreting observed rotational splittings of slowly-rotating red giant stars
Asteroseismology with the space-borne missions CoRoT and Kepler provides a
powerful mean of testing the modeling of transport processes in stars.
Rotational splittings are currently measured for a large number of red giant
stars and can provide stringent constraints on the rotation profiles. The aim
of this paper is to obtain a theoretical framework for understanding the
properties of the observed rotational splittings of red giant stars with slowly
rotating cores. This allows us to establish appropriate seismic diagnostics for
rotation of these evolved stars. Rotational splittings for stochastically
excited dipolar modes are computed adopting a first-order perturbative approach
for two benchmark models assuming slowly rotating cores. For red
giant stars with slowly rotating cores, we show that the variation of the
rotational splittings of modes with frequency depends only on the
large frequency separation, the g-mode period spacing, and the ratio of the
average envelope to core rotation rates (). This leds us to propose a
way to infer directly from the observations. This method is
validated using the Kepler red giant star KIC 5356201. Finally, we provide a
theoretical support for the use of a Lorentzian profile to measure the observed
splittings for red giant stars.Comment: 15 pages, 15 figures, accepted for publication in A&
- …