The simultaneous detection of 22- 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 of symbiotic stars that contain a long-period Mira-type variable. Our observations 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 properties 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 the 22-GHz maser and free–free emission are aligned. Three mechanisms can reproduce the data with varying degrees of success. All three lead naturally to normal levels of maser emission in SiO and 321-GHz H₂O and anomalously weak OH and 22-GHz H₂O masers. In the most convincing model, UV radiation and a fast wind from the companion remove the Mira's envelope of dusty, molecular gas, leaving a relatively small cavity of dense, neutral material within a large, ionized nebula. Excitation temperatures suggest that 321-GHz masers are normally excited close to the Mira whilst 22-GHz masers are more remote; in R Aqr, therefore, the 22-GHz masers do not form under optimum conditions. Instead, we see weak and narrow lines that form closer to the Mira, consistent with our high-resolution maps.