Variations of the H2O maser emission of W51M in 1981-1998

The results of spectral monitoring of the maser source W51M carried out in the water-vapor line at 1.35 cm (22GHz) on the 22 m telescope of the Pushch ino Radio Astronomy Observatory in 1981-1998 are reported and interpreted. Long-term variations of the maser flux with a period of 12-13 years are fou nd. W51M may be a relating and simultaneously expanding toroidal cloud of g as and dust around a young star with a mass of the order of similar to 15M( circle dot), with numerous high-velocity jets of maser condensations flowin g out in two broad cones along the polar axis of the torus. A stellar wind with a velocity of about 2000-3000 km/s is responsible For the maser pumpin g. (C) 2001 MAIK "Nauka/Interperiodica".