159 research outputs found
Driving and damping mechanisms in hybrid pressure-gravity modes pulsators
We study the energetic aspects of hybrid pressure-gravity modes pulsations.
The case of hybrid beta Cephei-SPB pulsators is considered with special
attention. In addition to the already known sensitivity of the driving
mechanism to the heavy elements mixture (mainly the iron abundance), we show
that the characteristics of the propagation and evanescent regions play also a
major role, determining the extension of the stable gap in the frequency domain
between the unstable low order pressure and high order gravity modes. Finally,
we consider the case of hybrid delta Sct-gamma Dor pulsators.Comment: 7 pages, 9 figures, in the proceedings of the Helas II Conference:
"Helioseismology, Asteroseismology and MHD Connections", Goettingen, August
200
Hybrid gamma Doradus/delta Scuti Stars: Comparison Between Observations and Theory
Gamma Doradus are F-type stars pulsating with high order g-modes. Their
instability strip (IS) overlaps the red edge of the delta Scuti one. This
observation has led to search for objects in this region of the HR diagram
showing p and g-modes simultaneously. The existence of such hybrid pulsators
has recently been confirmed (Handler 2009) and the number of candidates is
increasing (Matthews 2007). From a theoretical point of view, non-adiabatic
computations including a time-dependent treatment of convection (TDC) predict
the existence of gamma Dor/delta Sct hybrid pulsators (Dupret et al. 2004;
Grigahcene et al. 2006). Our aim is to confront the properties of the observed
hybrid candidates with the theoretical predictions from non-adiabatic
computations of non-radial pulsations including the convection-pulsation
interaction.Comment: 3 pages, 3 figures, Poster at "Stellar Pulsation: challenges for
theory and observation", Santa Fe, June 200
Application of time-dependent convection models to the photometric mode identification in gamma Doradus stars
We apply the Time-Dependent Convection (TDC) treatment of Gabriel \cite{Gabriel1996} and Grigahcène et al. \cite{Grigahcene} to the photometric mode identification in gamma Dor stars. Comparison of our theoretical results with the observed amplitudes and phases of the star gamma Dor is presented. This comparison makes the identification of the degree l of its pulsation modes possible and shows that our TDC models better agree with observations than Frozen Convection (FC) models
The underlying physical meaning of the relation
Asteroseismology of stars that exhibit solar-like oscillations are enjoying a
growing interest with the wealth of observational results obtained with the
CoRoT and Kepler missions. In this framework, scaling laws between
asteroseismic quantities and stellar parameters are becoming essential tools to
study a rich variety of stars. However, the physical underlying mechanisms of
those scaling laws are still poorly known. Our objective is to provide a
theoretical basis for the scaling between the frequency of the maximum in the
power spectrum () of solar-like oscillations and the cut-off
frequency (). Using the SoHO GOLF observations together with
theoretical considerations, we first confirm that the maximum of the height in
oscillation power spectrum is determined by the so-called \emph{plateau} of the
damping rates. The physical origin of the plateau can be traced to the
destabilizing effect of the Lagrangian perturbation of entropy in the
upper-most layers which becomes important when the modal period and the local
thermal relaxation time-scale are comparable. Based on this analysis, we then
find a linear relation between and , with a
coefficient that depends on the ratio of the Mach number of the exciting
turbulence to the third power to the mixing-length parameter.Comment: 8 pages, 11 figures. Accepted in A&
A comprehensive asteroseismic modelling of the high-amplitude delta Scuti star RV Arietis
We present a comprehensive asteroseismic study of the double-mode
high-amplitude delta Scuti star HD 187642 (RV Arietis). The modelling includes
some of the most recent techniques: 1) effects of rotation on both equilibrium
models and adiabatic oscillation spectrum, 2) non-adiabatic study of radial and
non-radial modes, 3) relationship between the fundamental radial mode and the
first overtone in the framework of Petersen diagrams. The analysis reveals that
two of the observed frequencies are very probably identified as the fundamental
and first overtone radial modes. Analysis of the colour index variations,
together with theoretical non-adiabatic calculations, points to models in the
range of [7065,7245] K in effective temperature and of [1190, 1270] Myr in
stellar age. These values were found to be compatible with those obtained using
the three other asteroseismic techniques.Comment: accepted for publication in A&
Solar-like oscillations in massive main-sequence stars. I. Asteroseismic signatures of the driving and damping regions
Motivated by the recent detection of stochastically excited modes in the
massive star V1449 Aql (Belkacem et al., 2009b), already known to be a
Cephei, we theoretically investigate the driving by turbulent convection. By
using a full non-adiabatic computation of the damping rates, together with a
computation of the energy injection rates, we provide an estimate of the
amplitudes of modes excited by both the convective region induced by the iron
opacity bump and the convective core. Despite uncertainties in the dynamical
properties of such convective regions, we demonstrate that both are able to
efficiently excite modes above the CoRoT observational threshold and the
solar amplitudes. In addition, we emphasise the potential asteroseismic
diagnostics provided by each convective region, which we hope will help to
identify the one responsible for solar-like oscillations, and to give
constraints on this convective zone. A forthcoming work will be dedicated to an
extended investigation of the likelihood of solar-like oscillations across the
Hertzsprung-Russell diagram.Comment: 9 pages, 14 figures, accepter in A&
Age determination of the HR8799 planetary system using asteroseismology
Discovery of the first planetary system by direct imaging around HR8799 has
made the age determination of the host star a very important task. This
determination is the key to derive accurate masses of the planets and to study
the dynamical stability of the system. The age of this star has been estimated
using different procedures. In this work we show that some of these procedures
have problems and large uncertainties, and the real age of this star is still
unknown, needing more observational constraints. Therefore, we have developed a
comprehensive modeling of HR8799, and taking advantage of its gamma
Doradus-type pulsations, we have estimated the age of the star using
asteroseismology. The accuracy in the age determination depends on the rotation
velocity of the star, and therefore an accurate value of the inclination angle
is required to solve the problem. Nevertheless, we find that the age estimate
for this star previously published in the literature ([30,160] Myr) is
unlikely, and a more accurate value might be closer to the Gyr. This
determination has deep implications on the value of the mass of the objects
orbiting HR8799. An age around 1 Gyr implies that these objects are
brown dwarfs.Comment: 5 pages, 3 figures, accepted in MNRAS Letter
Spectrum Analysis of Bright Kepler Gamma Doradus Candidate Stars
Ground-based spectroscopic follow-up observations of the pulsating stars
observed by the Kepler satellite mission are needed for their asteroseismic
modelling. We aim to derive the fundamental parameters for a sample of 26 Gamma
Doradus candidate stars observed by the Kepler satellite mission to accomplish
one of the required preconditions for their asteroseismic modelling and to
compare our results with the types of pulsators expected from the existing
light curve analysis. We use the spectrum synthesis method to derive the
fundamental parameters like Teff, logg, [M/H], and vsini from newly obtained
spectra and compute the spectral energy distribution from literature photometry
to get an independent measure of Teff. We find that most of the derived Teff
values agree with the values given in the Kepler Input Catalogue. According to
their positions in the HR-diagram three stars are expected Gamma Dor stars, ten
stars are expected Delta Sct stars, and seven stars are possibly Delta Sct
stars at the hot border of the instability strip. Four stars in our sample are
found to be spectroscopic binary candidates and four stars have very low
metallicity where two show about solar C abundance. Six of the 10 stars located
in the Delta Sct instability region of the HR-diagram show both Delta Sct and
Gamma Dor-type oscillations in their light curves implying that Gamma Dor-like
oscillations are much more common among the Delta Sct stars than predicted by
theory. Moreover, seven stars showing periods in the Delta Sct and the Delta
Sct-Gamma Dor range in their light curves are located in the HR-diagram left of
the blue edge of the theoretical Delta Sct instability strip. The consistency
of these findings with recent investigations based on high-quality Kepler data
implies the need for a revision of the theoretical Gamma Dor and Delta Sct
instability strips.Comment: 10 pages, 3 figures, 7 tables; accepted for publication in MNRA
Calibration of the pre-main sequence RS Cha binary system
Context: The calibration of binary systems with accurately known masses
and/or radii provides powerful tools to test stellar structure and evolution
theory and to determine the age and helium content of stars. We study the
eclipsing double-lined spectroscopic binary system RS Cha, for which we have
accurate observations of the parameters of both stars (masses, radii,
luminosities, effective temperatures and metallicity). Aims: We have calculated
several sets of stellar models for the components of the RS Cha system, with
the aim of reproducing simultaneously the available observational constraints
and to estimate the age and initial helium abundance of the system. Methods:
Using the CESAM stellar evolution code, we model both components starting from
the initial mass and metallicity and adjusting the input parameters and physics
in order to satisfy the observational constraints. Results: We find that the
observations cannot be reproduced if we assume that the abundance ratios are
solar but they are satisfied if carbon and nitrogen are depleted in the RS Cha
system with respect to the Sun. This is in accordance with the abundances
observed in other young stars. The RS Cha system is in an evolutionary stage at
the end of the PMS phase where models are not strongly sensitive to various
physical uncertainties. However we show that the oscillations of these two
stars, which have been detected, would be able to discriminate between
different options in the physical description of this evolutionary phase.Comment: 9 pages, 10 figures, accepted by Astronomy & Astrophysic
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