# CAOSP abstracts, Volume: 27, No.: 3, year: 1998

*Author(s):* AUDARD, N.; KUPKA, F.; MOREL, P.; PROVOST, J.; WEISS, W.W.
*Journal:* Contributions of the Astronomical Observatory Skalnate Pleso,
vol. 27, no. 3, p. 304-308.
*Date:* 04/1998
*Title:* Atmospheric structure and acoustic cut-off frequency of roAp stars
*Keyword(s):* STARS: ATMOSPHERES, CHEMICALLY PECULIAR, OSCILLATIONS,
INDIVIDUAL: HD 24712, HD 128898, HD 134214, VARIABLES: RAPIDLY OSCILLATING
*Pages:* 304 -- 308

**Abstract:**
Some of the rapidly oscillating (CP2) stars, have frequencies which are
larger than the theoretical acoustic cut-off frequency. As the cut-off
frequency depends on the T(tau) relation in the atmosphere, we have computed
models and adiabatic frequencies for pulsating Ap stars with T(tau) laws based
on Kurucz model atmospheres and on Hopf's purely radiative relation.
The frequency-dependent treatment of radiative transfer as well as an
improved calculation of the radiative pressure in Kurucz model atmospheres
increase the theoretical acoustic cut-off frequency by about 200 microHz,
which is closer to the observations. For alpha Cir we find models with
Kurucz atmospheres which have indeed a cut-off frequency beyond the largest
observed frequency and which are well within the T(eff) - L 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 would have an
acoustic cut-off frequency large enough. 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. However,
the observational errors for the astrophysical fundamental parameters
have to be reduced further and the model atmospheres refined.

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