THE SIMULTANEOUS DETECTION OF 22-GHZ AND 321-GHZ H2O MASER EMISSION TOWARDS THE SYMBIOTIC MIRA-R AQUARII

We report high spatial and spectral resolution measurements of masers towards R Aqr and H1-36, both of which are examples of the sub-class o f symbiotic stars that contain a long-period Mira-type variable. Our o bservations have resulted in the first detection of 321-GHz H2O maser action towards a symbiotic Mira - R Aqr. Comparison with simultaneous 22-GHz H2O maser data suggests that the masers do not have the same pr operties as those in the circumstellar envelopes of field Miras. R Aqr 's 22-/321-GHz peak flux density and luminosity ratios are low, as is the line width ratio. Continuum and spectral-line maps indicate that t he 22-GHz maser and free-free emission are aligned. Three mechanisms c an reproduce the data with varying degrees of success. All three lead naturally to normal levels of maser emission in SiO and 321-GHz H2O an d anomalously weak OH and 22-GRz H2O masers. In the most convincing mo del, UV radiation and a fast wind from the companion remove the Mira's envelope of dusty, molecular gas, leaving a relatively small cavity o f dense, neutral material within a large, ionized nebula. Excitation t emperatures suggest that 321-GHz masers are normally excited close to the Mira whilst 22-GHz masers are more remote; in R Aqr, therefore, th e 22-GHz masers do not form under optimum conditions. Instead, we see weak and narrow lines that form closer to the Mira, consistent with ou r high-resolution maps.