Determining the period of the long-period activity of the water-vapor maser in W75N

Abstract

We present the results of our study of the H₂O maser emission from the source W75N, which is associated with a star-forming region, between November 1994 and March 1999. The observations were carried out with the RT-22 radio telescope of the Pushchino Radio Astronomy Observatory (Lebedev Physical Institute). The maser emission in 1994–1999 can be represented as a superposition of flares of separate components with a duration from two to six months, which occurred mainly in the radial-velocity range 8–17.5 km s⁻¹. We detected a regular drift of the velocity centroid from 13 to 9 km s⁻¹ and an abrupt change in its velocity from 9 to 5 km s⁻¹, which took place at the initial stage of maser activity. Based on the variability of the total H₂O flux in all years of our observations of W75N (from December 1979 through March 1999), we conclude that the long-period variability of the water-vapor maser emission has a period of ∼11.5 years. We give arguments that this variability is mainly associated with the most compact group of maser spots, whose positions coincide with the position of the continuum source VLA 2.