MAGNETIC EFFECTS ON OSCILLATIONS IN ROAP STARS

We calculate the effect of a simple dipole magnetic held 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 st ars. We did not account for the field in the static models. Some of th e modes we calculate exhibit a strong driving due to the kappa-mechani sm acting in the hydrogen ionization zone. This driving is only somewh at 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. A t KG photospheric intensity, the mode frequencies are shifted by about 10-20 mu Hz from their nonmagnetic values. Such shifts are comparable to the small separations. Damping rates due to Alfvenic wave losses a re in the 2-10 mu Hz range and are comparable to nonadiabatic damping rates. Surface amplitudes significantly depart from pure, single spher ical harmonic dependence, which severely complicates mode identificati on and observational determination of large separations. Thus, taking into account the effects of the magnetic field is a prerequisite to an y meaningful roAp star asteroseismology and to understanding mode sele ction in these objects.