ИСТИНА

We report long-term observations of H2O and OH maser emission sources at wavelengths of 1.35 and 18 cm associated with circumstellar envelopes of late-type variable stars. The observations were carried out, respectively, on the 22-metre radio telescope of the Pushchino Radio Astronomy Observatory, Russia, and on the radio telescope of the Nançay Radio Astronomy Station, France. A sample of ~70 late-type long-period variable stars has been monitored in the 1.35-cm H2O line in 1980-2016. It includes Mira-type stars (U Ori, RS Vir, U Her, R Cas,…) and semiregular variables (R Crt, RT Vir, W Hya, VX Sgr,…) We have traced H2O maser variations and found them to follow the optical brightness variations with a time delay of 0.3-0.4 stellar period. Some strong H2O flares were observed, which probably occur every 10 to 15 periods, probably due to long-term changes in the mass-loss rate; good examples are U Ori, U Her, and RS Vir. Especially spectacular flares were demonstrated by the semiregular star W Hya, when its H2O peak flux density reached several thousand janskys, while on the average it did not exceed 50-100 Jy. The model of shock excitation of the H2O maser in combination with stellar radiation pumping is considered. In 2007-2016 the same sample of long-period variable stars was observed in the 18-cm OH lines. For 53 of them, the emission was detected in at least one of three OH lines (1612, 1665, or 1667 MHz). Circular and linear polarization of the maser emission was measured, yielding all four Stokes parameters. Polarized emission features were detected in the OH line spectra of 41 stars. A summary of all the observations is given. Features probably due to Zeeman splitting were detected in the OH line profiles of several stars. Estimated magnetic-field strengths in the maser sources are a few milligauss. Variability of the polarization characteristics of the maser emission of the stars on time intervals of several months was found. In particular, we discuss the data of 2007-2016 on the OH emission of the unique symbiotic star V627 Cas (AS 501, AFGL 2999) displaying maser emission in three ground-sate OH lines 1612, 1665, and 1667 MHz, with interesting redistribution with time of their relative intensities.