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

¹ Instituto Nacional de Astrofísica, Óptica y Electrónica, Luis Enrique Erro No. 1, Apdo Postal 51 y 216, 72840 Tonantzintla, Puebla, México e-mail: [lekht;mend]@inaoep.mx
² Sternberg Astronomical Institute, 13 Universitetskij prospekt, Moscow, 119992 Russia e-mail: gmr@sai.msu.ru
³ Pushchino Radio Astronomy Observatory, Astrospace Center of the Lebedev Institute of Physics, Russian Academy of Sciences, Pushchino, Moscow Region, 142290 Russia e-mail: tolm@prao.psn.ru

Received: 3 January 2005
Accepted: 3 February 2005

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.