Abstract

Amplitude modulation in the ZZ ceti star GD 244

Margit Paparo (Konkoly Observatory)

Zs. Bognár and Á. Sódor (Konkoly Observatory, Budapest, Hungary and Royal Observatory of Belgium, Brussels, Belgium), L. Molnár and E. Plachy (Konkoly Observatory, Budapest, Hungary)

There is a well-known general trend observed in the ZZ Ceti (or DAV) stars that towards the red edge of the instability strip we see more complex, non-sinusoidal light variations with longer periods and larger amplitudes. Short-term amplitude variations are also common among the cool ZZ Ceti stars. GD 244 is located in the middle of the DAV instability strip. Previous studies, based on observations covering several years, revealed five pulsation frequencies, two doublets and three single periods, by the light variations of the star. One of them (at 203 s) found to be considerably stable, and as part of a survey, utilized to search for a possible low mass companion applying the O-C technique (Mullally et al. 2008, ApJ, 676, 573). Although, they could not reveal the presence of such an object, the data obtained between 2003 and 2006, and our additional measurements in 2006 and 2007 at the Konkoly Observatory, allow the investigation of the long-term pulsational behaviour of GD 244. We found that while the 203 s mode remains stable both in frequency and amplitude, the 307.1 s period component of one of the doublets show clear long-term, periodic amplitude modulation, which can be fitted with a sine function having ~730 day period and ~7 mma amplitude. Neither the other doublet component (at 306.6 s) nor any of the other doublet's components (at ~256 s) show similar variations. One possible explanation is that two modes of fairly close frequencies are excited at ~307.1 s. We can detect indeed a corresponding peak in the Fourier spectrum of the combined yearly data. The other possibility is that a yet unknown physical mechanism, operating in the time scale of years, cause a periodic change of the energy content of the 307.1 s mode. This is the first time that such long-term periodic amplitude modulation was detected in a ZZ Ceti star.

Mode of presentation: poster