Frequency Analysis of the Rapidly Oscillating Ap Star HD 60435

Abstract

The cool Ap star HD 60435 was monitored in a programme of rapid B photometry during 18 nights in January/February 1984, from two stations widely spaced in longitude (the University of Toronto 0.6-m telescope at the Carnegie Southern Observatory (CARSO) on Las Campanas, Chile, and the 0.5-m telescope of the South African Astronomical Observatory ( SAAO)). On six of those nights, contiguous light curves from both sites were obtained.

Fourier analysis of these data confirms the rapid variability first reported by Kurtz (1984) and reveals several additional transient oscillations. HD 60435 exhibits persistent - but modulated - oscillations at a frequency near 1.4 IRHZ (period = 11.9 minutes), and short-lived oscillations at frequencies near 1.1 and 4.2 mHz (periods of 15.2 and 4.0 minutes, respectively). These latter two periods represent the longest and shortest yet observed in the class of rapidly oscillating Ap stars.

We have applied the oblique pulsator model (Kurtz 1982) to the fine-scale splittings detected in the frequency spectra of the 1.4 and 1.1 mHz oscillations. Also, the series of frequencies close to 1.4 mHz which fall into a pattern of roughly equal spacing is compared to such spacings predicted for overtones in pulsating main-sequence A stars (Shibahashi and Saio 1984). Both approaches suggest that HD 60435 is undergoing non-radial pulsations of odd and even degree (probably with ℓ ≲ 3).

The oblique pulsator interpretation of the splittings in the frequency spectrum and the amplitude modulation of the 1.4 mHz oscillations also predict: a rotation period of approximately eight days for this star. Mean photometry of HD 60435, collected by the authors, supports a similar value of 7.7 days for the period.

Analysis of the oscillations is hampered by ambiguities due to daily aliases present in the data, and by the complicated structure and time-dependence of the frequency spectrum. Further observations of HD 60435 are essential if we are to fully understand its rapid variations.