Cyclic activity of the water-vapor maser in S128

Abstract
The results of observations of the S128 H2O maser carried out from February 1995 to March 2001 on the 22-m radio telescope of the Pushchino Radio Astronomy Observatory are presented. Two activity cycles of the H2O maser with a period of about 10 years were observed during the total monitoring interval (1981–2001). This may be connected either with cyclic activity of the central star in S128 during its formation or with the influence on the H2O masering region of shocks arising near an ionization front at the interface of two colliding CO clouds. The emission at radial velocities from −73 to −70 km/s consists of four emission features. The emission feature at −71.8 km/s exhibits a flux dependence on linewidth that is typical of an unsaturated maser.

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