OH and H 2 O maser variations in W33B

Description

Context. The active star-forming region W33B is a source of OH and H 2 O maser emission located in distinct zones around the central object. Aims. The aim was to obtain the complete Stokes pattern of polarised OH maser emission and to trace its variability and to investigate flares and long-term variability of the H 2 O maser and evolution of individual emission features. Methods. Observations in the OH lines at a wavelength of 18 cm were carried out on the Nançay radio telescope (France) at a number of epochs in 2008−2014; H 2 O line observations (long-term monitoring) at λ = 1.35 cm were performed on the 22 m radio telescope of the Pushchino Radio Astronomy Observatory (Russia) between 1981 and 2014. Results. We have observed strong variability of the emission features in the main 1665-and 1667 MHz OH lines as well as in the 1612 MHz satellite line. Zeeman splitting has been detected in the 1665 MHz OH line at 62 km s −1 and in the 1667 MHz line at 62 and 64 km s −1. The magnetic field intensity was estimated to be from 2 to 3 mG. The H 2 O emission features form filaments, chains with radial-velocity gradients, or more complicated structures including large-scale ones. Conclusions. Long-term observations of the hydroxyl maser in the W33B region have revealed narrowband polarised emission in the 1612 MHz line with a double-peak profile characteristic of type IIb circumstellar masers. The 30 year monitoring of the water-vapour maser in W33B showed several strong flares of the H 2 O line. The observed radial-velocity drift of the H 2 O emission features suggests propagation of an excitation wave in the masering medium with a gradient of radial velocities. In OH and H 2 O masers some turbulent motions of material are inferred.

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