The acoustic cut-off frequency of roAp stars ?

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 T() relation, we have computed models and adiabatic frequencies for pulsating Ap stars with more realistic atmospheres which in- clude a frequency dependent treatment of radiative transfer, take blanketing effects into account, and which have a better treat- ment of the radiative pressure. In addition, we are using opac- ity distribution functions specific to the atmospheric composi- tion. With these improvements over the classical stellar mod- els the theoretical acoustic cut-off frequency for roAp stars are increased by about 200 Hz, which brings them close to the observations. We restrict the comparison of our computations with obser- vations to those two 'pathological' roAp stars for which more reliable astrophysical parameters are available, HD 24712 and Cir, and comment briefly on a third one, HD 134214. For Cir we find models which have indeed a cut-off frequency beyond the largest observed frequency and which are well within the Te - L=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 derived by spectroscopy would have an acoustic cut-off frequency large enough. HD 134214 fits our models best, however, the error box for Te - L=L 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.