Short time-scale frequency and amplitude variations in the pulsations of an roAp star: HD 217522

Abstract

Photometric observations of HD 217522 in 1981 revealed only one pulsation frequency ν1 = 1.215 29 mHz. Subsequent observations in 1989 showed the presence of an additional frequency ν2 = 2.0174 mHz. New observations in 2008 confirm the presence of the mode with ν2 = 2.0174 mHz. Examination of the 1989 data shows amplitude modulation over a time-scale of the order of a day, much shorter than what has been observed in other rapidly oscillating Ap (roAp) stars. High spectral and time resolution data obtained using the Very Large Telescope in 2008 confirm the presence of ν2 and short-term modulations in the radial velocity amplitudes of rare earth elements. This suggests growth and decay times shorter than a day, more typical of solar-like oscillations. The driving mechanism of roAp stars and the Sun are different, and the growth and decay seen in the Sun are due to stochastic nature of the driving mechanism. The driving mechanism in roAp stars usually leads to mode stability on a longer time-scale than in the Sun. We interpret the reported change in ν1 between the 1982 and 1989 data as part of the general frequency variability observed in this star on many time-scales.