ИСТИНА

Results of long-term studies of circumstellar molecular maser emission of late-type giant and supergiant stars are reported. The observations have been carried out during several decades in the H2O line at a wavelength of 1.35 cm (22-meter telescope in Pushchino, Russia) and in the hydroxyl (OH) lines at 18 cm (Nançay radio telescope, France). A sample of ~70 AGB long-period variable stars has been monitored in the 1.35-cm H2O line in 1980-2018. 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 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 discussed. The same sample of AGB 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. 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. In particular, we discuss the data of 2007-2018 on the OH emission of the unique symbiotic star V627 Cas (AS 501, AFGL 2999) displaying maser emission in three ground-state OH lines 1612, 1665, and 1667 MHz, with remarkable redistribution with time of their relative intensities, as well the new data on the OH and H2O emission of the post-AGB star AI CMi.