1998A&A...335..954A

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(τ) 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 200µHz, 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, HD24712 and αCir, and comment briefly on a third one, HD134214. For αCir we find models which have indeed a cut-off frequency beyond the largest observed frequency and which are well within the T_eff-L/L_☉ error box. For HD24712 only models which are hotter by about 100K 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. HD134214 fits our models best, however, the error box for T_eff - 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.