EFFECTS OF THE QUADRUPOLE COMPONENT OF MAGNETIC-FIELDS ON THE RAPID OSCILLATIONS OF AP STARS

We interpret the septuplet fine structure in the power spectrum of a r apidly oscillating Ap star, HR 3831, as a manifestation of the nonradi al dipole mode distorted by the magnetic field and the rotation of the star. Taking into account both the dipole and quadrupole components o f the magnetic field and using a reasonable theoretical stellar model of an A-type star, we show that the axisymmetric dipole mode in the ca se of a nonmagnetic nonrotating star is distorted to have l = 0, 1, 2, 3, 4, and 5 components, and that the observed septuplet structure can be well explained as a part of the Ii-fold fine structure of the indu ced components. We derive the relations among the amplitudes of the pe aks in the fine structure, and show that various parameters of rapidly oscillating Ap stars can be determined by analyzing the fine structur e.