A frequency analysis of new observations of the rapidly oscillating Ap star HD 217522: evidence for mode-switching

Abstract

Photoelectric photometry of the rapidly oscillating Ap (roAp) star HD 217522 was obtained at Cerro–Tololo Inter-American Observatory (CTIO), European Southern Observatory (ESO), Perth Observatory and South African Astronomical Observatory (SAAO) in 1989. We present the results of a frequency analysis of these data, all of which were obtained in Johnson B except for the ESO data, which are Strömgren uvby. Our analysis reveals a significant shift of the primary oscillation frequency from |$\nu_1(1982) = 1.21529\pm 0.00004\enspace \text{mHz} \enspace \text {to}\enspace \nu_1(1989) = 1.1999\pm 0.0001 \enspace \text{mHz}$|⁠. We attribute this frequency change to a change of pulsation mode. In both the 1982 and 1989 data there is no apparent regularity to the amplitude modulation of ν1, suggesting that this modulation is not periodic, but rather is caused by short growth and decay times of the pulsation mode. A second frequency, |$\nu_2 =2.0174 \pm 0.0001 \enspace\text {mHz}$|⁠, is present on many nights in the new data. This frequency is entirely absent from the 1982 data and thus provides additional evidence for mode growth times which are shorter than the total time-span of the observations. In addition, the amplitude modulation of the frequencies ν1 and ν2 do not seem to be correlated. We use the separation |$\nu_1(1982) - \nu_1(1989) = 15.4\pm 0.1 \enspace \mu\text {Hz}$| to estimate the luminosity of HD 217522. Transient frequencies are present in some of our data from CTIO and ESO. One of those frequencies at 4.18 mHz has previously been seen in data obtained at ESO on HR 1217 and also in data obtained at Las Campanas on HD 60435; we argue that these 4.18-mHz frequencies are artifacts caused by poorly maintained equipment at the Chilean observatories. The change of the principal pulsation mode in HD 217522 sometime during the last seven years in which no observations were obtained suggests that the roAp stars should be monitored more frequently to watch for such changes.