Periodicities and regulatory mechanisms in Lyrae

Abstract

We utilize 210 years of β Lyrae eclipse timings [(O-C)s] and 100 years of radial velocities to search for eccentricity and apsidal motion, and for periodic variations. No (O-C) periods pass standard significance tests, but we find eight significant radial velocity periods by independent methods of time series analysis. Waveforms of the eight periodicities are readily seen in phase plots, with most not being simple sinusoids. The causes are not known, but could be orbiting stars. Inspection of the few existing polarimetric observations at two well-separated epochs does not find the radial velocity periods reproducibly. We looked thoroughly for direct evidence of eccentricity, but without success. Eccentricity that is approximately constant and as large as 0.010 seems ruled out, but e almost as large as this is possible, especially if it is variable on short time-scales. Apsidal motion could be a sensitive indicator of small e, but large gaps in the radial velocity record make apsidal motion detection difficult. We searched by three methods, but found only a few doubtful possibilities for apsidal periods. We also investigated several mechanisms for the previously known 283-d light-curve period by estimating ranges of plausible apsidal and precession periods. Apsidal periods are much too long to account for the 283-d variation, and precession is incompatible with existing photometry. Disc pulsation is so far not ruled out. Taking note of the uniquely constant orbital period change that has persisted at a consistent +19 s per year for 210 yr, we propose a feedback mechanism to regulate period change via impulsive mass transfer near periastron. The mechanism involves a generalization of the theory of period changes induced by lobe overflow so as to include impulsive overflow, and operates through a (perhaps unobservably) small and variable eccentricity.