A comparison of OH and H2O maser properties of Mira and semiregular variable stars

Abstract

A sample of 17 Miras and nine semiregular variable stars was simultaneously observed in the OH and H2O maser lines with sensitivities of 0.15 and 0.6 Jy, respectively. One new H2O source was detected. Among Miras the H2O maser luminosity is well correlated with the OH maser luminosity. The ratio of H2O to OH maser luminosities decreases by about four orders of magnitude from the semiregulars to the Miras as a result of an increase in the mass-loss rate. Some semiregulars have maser properties typical of Mira stars. H2O emission is most likely to be detected from Miras at velocities close to the stellar velocity, while emission at blueshifted velocities dominates the detections from semiregulars. OH emission from the semiregulars usually occurs at greater redshifts and blueshifts with respect to the stellar velocities than OH emission observed from the Miras. The differences in the average composite maser profiles in both groups of stars can be related to the velocity gradients in the maser regions. It is suggested that some semiregular variables with maser emission may be in a later stage of stellar evolution than the Miras.