Astron. Astrophys. Suppl. Ser. 146, 385-395

Variability of the H₂O maser associated with U Orionis

G.M. Rudnitskij¹ - E.E. Lekht² - J.E. Mendoza-Torres² - M.I. Pashchenko¹ - I.I. Berulis^3,4

Send offprint request: G.M. Rudnitskij

1 - Sternberg Astronomical Institute, 13 Universitetskij prospekt, Moscow 119899, Russia
e-mail: gmr, pachenko@sai.msu.ru
2 - Instituto Nacional de Astrofísica, Óptica y Electrónica, Luis Enrique Erro No. 1, Apdo Postal 51 y 216, 72840 Tonantzintla, Puebla, Mexico
e-mail: lekht, mend@inaoep.mx
3 - Pushchino Radio Astronomy Observatory, Astrospace Center of the Lebedev Institute of Physics, Russian Academy of Sciences, Pushchino, Moscow Region 142292, Russia
e-mail: berulis@prao.psn.ru
4 - Kaunas University of Technology, K. Donelaicio g. 73, 3006 Kaunas, Lithuania

Received December 3, 1999; accepted August 11, 2000

Abstract:

H₂O line observations at $\lambda=1.35$ cm of the Mira Ceti-type variable star U Ori are reported. The observations cover the time interval from March, 1980, to September, 1999. Variations of the integral flux and velocity centroid of the H₂O line are analysed. The flux in general correlates with the visual light curve, following it with some phase delay $\Delta\varphi\sim0.2-0.4P$ (P is the period of the star). The maser emission is generated in a quasi-stationary layer of gas and dust at a distance of about 10¹⁴ cm from the stellar centre. The maser variability is explained by the action of periodic shocks, driven by stellar pulsation and arriving to the maser in each stellar cycle. The shocks provide the maser pumping, whereas the sink of the waste energy is controlled by the dust, periodically heated by the stellar radiation near the light maximum; this accounts for the correlation of the maser radiation maximum with the descending branch of the light curve. Temporary weakness of the maser emission may be due to decay of the quasi-stationary layer, which is then rebuilt by a powerful shock, carrying away from the star a portion of the lost mass, once per a few stellar periods - the "superperiod''. In its turn, the superperiod may reflect multiperiodic pulsation of the star or the presence of a long-term activity cycle, connected with restructuring of the stellar magnetic field, which is known to be strong in U Ori.

Key words: stars: variables: Miras -- circumstellar matter -- stars: individual: U Ori -- radio lines: stars -- masers -- shock waves