Variability of the H₂O maser associated with the M-supergiant S Persei

Abstract

We present the results from observing the circumstellar maser emission of the M-type supergiant S Per in the 616–523 water-vapour line at 1.35 cm. The observations were carried out in 1981–2002 (–2 452 480) on the RT-22 radio telescope of the Pushchino Radio Astronomy Observatory, Astrospace Center of the Lebedev Institute of Physics, Russian Academy of Sciences. The H2O spectra obtained represent an unprecedented long, uniform dataset on this star. We discuss the properties of the optical and maser variations of S Per, together with particulars of the available VLBI maps. The close relation between maser and optical variations favors a model in which mass-loss is episodic. Changes observed in the total H2O line flux follow the visual light curve with a delay of 0.01 to 0.5P, where d is the mean light cycle for S Per. The feature at km s-1 flared in July 1988, which seemed to be the response of the maser to an unusually bright optical maximum. The position of the -44-km s-1 feature on the VLBI maps coincides with the direction toward the optical stellar disc, which can be explained by amplification of enhanced stellar continuum by the H2O line.