Magnetic effects on oscillations in roAp stars

We calculate the effect of a simple dipole magnetic field on high-order p-mode oscillations. The stellar models, oscillation modes, and range of field intensity were selected to correspond to the data on roAp stars. We did not account for the field in the static models. Some of the modes we calculate exhibit a strong driving due to the κ-mechanism acting in the hydrogen ionization zone. This driving is only somewhat smaller than the radiative damping occurring beneath. We argue that the situation is likely to reverse after needed improvements are made in model calculations. The effect of the field is very significant. At KG photospheric intensity, the mode frequencies are shifted by about 10-20 μHz from their nonmagnetic values. Such shifts are comparable to the small separations. Damping rates due to Alfvénic wave losses are in the 2-10 μHz range and are comparable to nonadiabatic damping rates. Surface amplitudes significantly depart from pure, single spherical harmonic dependence, which severely complicates mode identification and observational determination of large separations. Thus, taking into account the effects of the magnetic field is a prerequisite to any meaningful roAp star asteroseismology and to understanding mode selection in these objects.