Time Variations of Stellar Water Masers

The 22-GHz H2O spectra of the stars RS Vir, RT Vir, R Aql, W Hya, U Her, S Cr B, Rx Boo, R Crt and VY CMa have been observed at intervals during the period 1974 September–1977 May. Optical and infrared measurements have also been made. New components have been observed in the H2O spectra of most of the stars, and the flux density of W Hya reached 2000 Jy near JD 2442700. The intensities of the three main groups of components in VY CMa varied in phase, consistent with a central pump source. In several stars the intensities were very different from those found by earlier observers, showing that stellar H2O masers are often not stable for more than a few cycles of the stellar luminosity. For part of the time the H2O and infrared intensities of R Aql and RS Vir were anticorrelated. Observations of correlation over fairly short periods of time do not necessarily imply good correlation over more extended periods. The effectiveness of infrared radiation in pumping H2O masers is open to question, owing to the conflicting predictions of the available theoretical analyses. Our observations suggest that collisional processes are involved in pumping some stellar masers. Dust in the atmospheres of late-type stars may also play an important part.