The high-overtone p-mode spectrum of the rapidly oscillating Ap star HR 1217 (HD 24712) - results of a frequency analysis of 324 hr of multi-site photometric observations obtained during a 46-d time-span in 1986

Abstract

We present new high-speed photometric observations of the rapidly oscillating Ap star HR 1217 obtained during 365 hr of observation at eight observations in 1986 October, November and December. We show that the 6-min light variations are amplitude modulated in phase with the magnetic variations, with amplitude maximum and magnetic maximum coinciding; mean light minimum occurs at a slightly, but probably significantly, different time. A frequency analysis of 324 hr of those observations over a 46-d time-span yields six principal frequencies, ali of which are amplitude modulated with the rotation of the star. The duty cycle for this dataset is 29 per cent; the 3 d -l (34 μHz) aliases, which have previously caused the failure of frequency analyses of data obtained at a single site, are only 12 per cent in amplitude of the central peak of the spectral window and cause little problem. We find: (i) HR 1217 is an oblique rotator with a centred dipole magnetic field. ( ii) HR 1217 is an oblique pulsator with the pulsation axis and the magnetic axis aligned. (iii) There are six principal frequencies of pulsation in HR 1217. We find that v2 and v4 are dipole modes (l = 1, m =O); v3 and v5 cannot be described by single spherical harmonics - they look similar to dipole modes, but they have amplitude mínima which are lower than expected. (iv) lt is not possible to discriminate between the hypothesis that v 1 , v2 , v3, v4 and v5 are basically due to alternating even and odd l-modes with v0 =68 μHz, and the hypothesis that they are basícally due to dipole modes with v0 = 34 μHz. (v) The secondary frequencies indicate that the principal frequencies are amplitude modulated on a time-scale of months. This is not likely to be due to pulsation in higher degree modes, and hence indicates that the amplitudes of the principal frequencies are not completely stable. The frequency spectrum of HR 1217 is qualitatively similar to that of the sun and presents a wealth of intriguing problems.