Observations of late-type variable stars in the water-vapor radio line. The supergiant VX Sagittarii

Observations of the circumstellar maser emission of the M supergiant VX Sgr in the water-vapor line at 1.35 cm are presented. The observations were ca rried out from 1981-1998 (JD 2 444 655-2 450 966) on the 22-m radio telesco pe of the Pushchino Radio Astronomy Observatory of the Astro Space Center o f the Lebedev Institute of Physics. Throughout the 17 years of the observat ions, there were two groups of emission features in the H2O-line profile, w hich originate in the two oppositely directed lobes of a bipolar outflow fr om the star. A redistribution of the integrated flux F-int between the two groups of features was noted: in 1981-1987, the group with negative velocit ies (V-LSR < V-*, where V-* is the stellar velocity) dominated; starting fr om 1993, F-int for the features with V-LSR > V-* slightly exceeded that for features with V-LSR < V-*. This redistribution of F-int in the H2O-line pr ofile may be associated with a change in the dominant direction for the bip olar outflow due to restructuring of the overall dipolar magnetic field of VX Sgr. A model for the VX Sgr H2O maser source with a circumstellar disk a nd bipolar outflow in two cones with half-opening angle theta similar to 60 degrees is discussed. The axis of the bipolar outflow also forms an angle i similar to 60 degrees to the line of sight. The estimated bipolar-outflow expansion velocity V-0 in the H2O-maser region (R = (1.5-5) x 10(15) cm) i s similar to 10 km/s. The variability of the H2O maser is correlated with t he visual light curve of VX Sgr. However, the phase delay Delta phi of the F-int(H2O) variations relative to the optical variations changed form 0 to similar to 1 stellar period (P = 732(d)) over the time covered by the maser observations. If the variability of the H2O-maser source is the result of periodic impacts of shock waves driven by stellar pulsations, the travel ti me for the shock from the photosphere to the inner boundary of the H2O mase r shell may be as long as (10-15)P.