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Astron. Astrophys. 335, 954-958 (1998)

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The acoustic cut-off frequency of roAp stars *

N. Audard ** 1, 2, 3, F. Kupka 1, P. Morel 2, J. Provost 2, and W.W. Weiss 1

1 Institute for Astronomy, University of Vienna, Türkenschanzstraße 17, A-1180 Vienna, Austria (last_name@galileo.ast.univie.ac.at)
2 Département Cassini, UMR 6529, Observatoire de la Côte d'Azur, BP 4229, F-06304 Nice Cedex 4, France (last_name@obs-nice.fr)
3 Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 OHA, UK (last_name@ast.cam.ac.uk)

Received 6 May 1997 / Accepted 4 April 1998

Abstract

Some of the rapidly oscillating (roAp) stars, have frequencies which are larger than the acoustic cut-off frequency determined from published stellar models which usually assume a grey atmosphere. As the cut-off frequency depends on the [FORMULA] relation, we have computed models and adiabatic frequencies for pulsating Ap stars with more realistic atmospheres which include a frequency dependent treatment of radiative transfer, take blanketing effects into account, and which have a better treatment of the radiative pressure. In addition, we are using opacity distribution functions specific to the atmospheric composition. With these improvements over the classical stellar models the theoretical acoustic cut-off frequency for roAp stars are increased by about 200 µHz, which brings them close to the observations.

We restrict the comparison of our computations with observations to those two `pathological' roAp stars for which more reliable astrophysical parameters are available, HD 24712 and [FORMULA] Cir, and comment briefly on a third one, HD 134214. For [FORMULA] Cir we find models which have indeed a cut-off frequency beyond the largest observed frequency and which are well within the [FORMULA] - [FORMULA] error box. For HD 24712 only models which are hotter by about 100 K and less luminous by nearly 10% than what is actually the most probable value derived by spectroscopy would have an acoustic cut-off frequency large enough. HD 134214 fits our models best, however, the error box for [FORMULA] - [FORMULA] is the largest of all three stars.

One may thus speculate that the old controversy about a mismatch between observed largest frequencies and theoretical cut-off frequencies of roAp star models is resolved.

Key words: stars: chemically peculiar – stars: oscillations – stars: individual: HD 24712; HD 128898; HD 134214

* Based on hipparcos data.
** Lise Meitner fellow of the Fonds zur Förderung der wissenschaftlichen Forschung

Send offprint requests to: N. Audard, Cambridge

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© European Southern Observatory (ESO) 1998

Online publication: June 26, 1998
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