Pulsational properties of 1.8 M$_{\odot}$ 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.
We compare the accurate empirical mass-luminosity (M-L) relation based on five Hyades binary systems to predictions of stellar models calculated with various input parameters (helium, metallicity, ...age) or physics (mixing-length ratio, model atmosphere, equation of state, microscopic diffusion). Models based on a helium content Ysim0.28 inferred from the dydz enrichment law are more than 3sigma beyond the observations, suggesting that the initial helium abundance is lower than expected from its supersolar metallicity. With the photometric metallicity (FeH=0.144pm0.013 dex, Grenon (2000) we derive Y=0.255\pm0.009. Because of the (Y,FeH) degeneracy in the M-L plane, the uncertainty grows to Delta Y=0.013 if the metallicity from spectroscopy is adopted (FeH=0.14pm0.05 dex, Cayrel de Strobel et al 1997). We use these results to discuss the Hertzsprung-Russell (HR) diagram of the Hyades, in the (Mv,B-V) plane, based on the very precise Hipparcos dynamical parallaxes. Present models fit the tight observed sequence very well except at low temperatures. In the low mass region of the HR diagram sensitive to the mixing-length parameter (aMLT), the slope of the main sequence (MS) suggests that aMLT could decrease from a solar (or even supersolar) value at higher mass to subsolar values at low mass, which is also supported by the modeling of the vB22 M-L relation. We find that the discrepancy at low temperatures (B-V\gtrsim 1.2) remains, even if an improved equation of state or better model atmospheres are used. Finally, we discuss the positions of the stars at turn-off in the light of their observed rotation rates and we deduce that the maximum age of the Hyades predicted by the present models is sim650 Myr.
The acoustic power injected by turbulent convection into solar-like
oscillations depends on the details of the turbulent spectrum. A theoretical
formulation for the oscillation power is developed ...which generalizes previous
ones. The formulation is first calibrated on a solar model in such a way as to
reproduce the solar seismic data. This allows to investigate different
assumptions about the stellar turbulent spectrum. We next explore consequences
of the assumed turbulent description for some potentially solar-like
oscillating stars. Large differences are found in the oscillation power of a
given star when using different turbulent spectra as well as in a star to star
comparison. Space seismic observations of such stars will be valuable for
discriminating between several turbulent models.
Acoustic power and oscillation amplitudes of radial oscillations computed for a solar model are compared with solar seismic observations. The oscillations are assumed stochastically excited by ...turbulence. The numerical computations are based upon a theoretical formulation of the power going into solar like oscillation modes as proposed by Samadi et al. (2000) in a companion paper. This formulation allows to investigate several assumptions concerning properties of the stellar turbulence. We find that the entropy source plays a dominant role in the stochastic excitation compared with the Reynold stress source in agreement with Goldreich et al. (1994). We consider several turbulent kinetic energy spectra suggested by different observations of the solar granulation. Differences between turbulent spectra manifest themselves by large differences in the computed oscillation powers at high oscillation frequency. Two free parameters which are introduced in the description of the turbulence enter the expression for the acoustic power. These parameters are adjusted in order to fit to the solar observations of the surface velocity oscillations. The best fit is obtained with the kinetic energy spectrum deduced from the observations of the solar granulation by Nesis et al. (1993); the corresponding adjusted parameters are found to be compatible with the theoretical upper limit which can be set on these parameters. The adopted theoretical approach improves the agreement between solar seismic observations and numerical results.
Asteroseismology of stars in clusters has been a long-sought goal because the assumption of a common age, distance and initial chemical composition allows strong tests of the theory of stellar ...evolution. We report results from the first 34 days of science data from the Kepler Mission for the open cluster NGC 6819 -- one of four clusters in the field of view. We obtain the first clear detections of solar-like oscillations in the cluster red giants and are able to measure the large frequency separation and the frequency of maximum oscillation power. We find that the asteroseismic parameters allow us to test cluster-membership of the stars, and even with the limited seismic data in hand, we can already identify four possible non-members despite their having a better than 80% membership probability from radial velocity measurements. We are also able to determine the oscillation amplitudes for stars that span about two orders of magnitude in luminosity and find good agreement with the prediction that oscillation amplitudes scale as the luminosity to the power of 0.7. These early results demonstrate the unique potential of asteroseismology of the stellar clusters observed by Kepler.
Solar-like oscillations are stochastically excited by turbulent convection.
In this work we investigate changes in the acoustic oscillation power spectrum
of solar-type stars by varying the treatment ...of convection in the equilibrium
structure and the properties of the stochastic excitation model. We consider
different stellar models computed with the standard mixing-length description
by Bohm-Vitense (1958) and with a generalized formulation of the mixing-length
approach by Gough (1976, 1977). We calculate the acoustic power generated by
the turbulent convection which is injected stochastically into the acoustic
pulsation modes.
Large differences in the oscillation powers are obtained depending on the
choice of the assumed convection formulation. We show that the high-quality
data Eddington will provide, will allow us to distinguish between theoretical
predictions of acoustic power spectra obtained with different convection
models.
Pulsational properties of 1.8 M\(_{\odot}\) 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.
The acoustic power injected by turbulent convection into solar-like oscillations depends on the details of the turbulent spectrum. A theoretical formulation for the oscillation power is developed ...which generalizes previous ones. The formulation is first calibrated on a solar model in such a way as to reproduce the solar seismic data. This allows to investigate different assumptions about the stellar turbulent spectrum. We next explore consequences of the assumed turbulent description for some potentially solar-like oscillating stars. Large differences are found in the oscillation power of a given star when using different turbulent spectra as well as in a star to star comparison. Space seismic observations of such stars will be valuable for discriminating between several turbulent models.
Using VLTI/VINCI angular diameter measurements, we constrain the evolutionary status of three asteroseismic targets: the stars \(\delta\) Eri, \(\xi\) Hya, \(\eta\) Boo. Our predictions of the mean ...large frequency spacing of these stars are in agreement with published observational estimations. Looking without success for a companion of \(\delta\) Eri we doubt on its classification as an RS CVn star.
Solar-like oscillations are stochastically excited by turbulent convection. In this work we investigate changes in the acoustic oscillation power spectrum of solar-type stars by varying the treatment ...of convection in the equilibrium structure and the properties of the stochastic excitation model. We consider different stellar models computed with the standard mixing-length description by Bohm-Vitense (1958) and with a generalized formulation of the mixing-length approach by Gough (1976, 1977). We calculate the acoustic power generated by the turbulent convection which is injected stochastically into the acoustic pulsation modes. Large differences in the oscillation powers are obtained depending on the choice of the assumed convection formulation. We show that the high-quality data Eddington will provide, will allow us to distinguish between theoretical predictions of acoustic power spectra obtained with different convection models.
