arXiv subjects

G. Houdek

Publications and source records attributed to G. Houdek.

58 records · Page 4Linked to original sources

arXiv2002

A study of convection with delta Scuti stars in open clusters: Pleiades

In this work we propose a preliminary seismic investigation of $δ$ Scuti stars in the Pleiades cluster, focusing on potential diagnostics of convection and core-overshooting. Taking into account the effect of fast rotation in the modelling, we compare observed frequencies for 4 $δ$ Scuti stars with radial linear instability predictions. A satisfying agreement is reached between the predicted ranges of unstable modes and those derived from observations for ``low-mass'' stars ($\sim 1.55 M_{\odot}$). However, a strong disagreement is found for ``high-mass'' stars ($\sim 1.77 M_{\odot}$), whatever the mixing length $(α)$ value. These results are compared with previous ones obtained for Praesepe.

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arXiv2001

Oscillations of alpha UMa and other red giants

There is growing observational evidence that the variability of red giants could be caused by excitation of global modes of oscillation. The most recent evidence of such oscillations was reported for alpha UMa by Buzasi et al.(2000). We address the problem of radial and nonradial mode excitation in red giants from the theoretical point of view. In particular, we present the results of numerical computations of oscillation properties of a model of alpha UMa and of several models of a 2M_sun star in the red-giant phase. In the red giant stars by far most of the nonradial modes are confined to to the deep interior, where they have the g-mode character. Only modes at resonant frequencies of the p-mode cavity have substantial amplitudes in the outer layers. We have shown that such modes can be unstable with the linear growth rates similar to those of corresponding to radial modes. We have been unable to explain the observed oscillation properties of alpha UMa, either in terms of mode instability or in terms of stochastic excitation by turbulent convection. Modes at the lowest frequencies, which exhibit the largest amplitudes and may correspond to the first three radial modes, are found stable if all effects of convection are taken into account. The observed frequency dependence of amplitudes does not agree with what one expects from stochastic excitation. The predicted fundamental mode amplitude is by about two orders of magnitude smaller than those of high frequency modes, which is in stark disagreement with the observations.

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arXiv1999

Amplitudes of stochastically excited oscillations in main-sequence stars

We present estimates of the amplitudes of intrinsically stable stochastically excited radial oscillations in stars near the main sequence. The amplitudes are determined by the balance between acoustical energy generation by turbulent convection (the Lighthill mechanism) and linear damping. Convection is treated with a time-dependent, nonlocal, mixing-length model, which includes both convective heat flux and turbulent pressure in both the equilibrium model and the pulsations. Velocity and luminosity amplitudes are computed for stars with masses between 0.9 M_\sun and 2.0 M_\sun in the vicinity of the main sequence, for various metallicities and convection parameters. As in previous studies, the amplitudes are found to increase with stellar mass, and therefore with luminosity. Amongst those stars that are pulsationally stable, the largest amplitudes are predicted for a 1.6 M_\sun model of spectral type F2; the values are approximately 15 times larger than those measured in the Sun.

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arXiv1995

NEAR-SURFACE EFFECTS IN MODELLING OSCILLATIONS OF ETA BOO

Following the report of solar-like oscillations in the G0 V star eta Boo (Kjeldsen et al. 1995, AJ 109, 1313), a first attempt to model the observed frequencies was made by Christensen-Dalsgaard et al. (1995, ApJ Letters, in press). This attempt succeeded in reproducing the observed frequency separations, although there remained a difference of about 10 microHz between observed and computed frequencies. In those models, the near-surface region of the star was treated rather crudely. Here we consider more sophisticated models that include non-local mixing-length theory, turbulent pressure and nonadiabatic oscillations.

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