2008A&A...477..185M

Maser emission from the H₂O molecule probes the warm, inner circumstellar envelopes of oxygen-rich red giant and supergiant stars. Multi-maser transition studies can be used to put constraints on the density and temperature of the emission regions. A number of known H₂O maser lines were observed toward the long period variables R Leo and W Hya and the red supergiant VY CMa. A search for a new, not yet detected line near 475GHz was conducted toward these stars. The Atacama Pathfinder Experiment telescope was used for a multi-transition observational study of submillimeter H₂O lines. The 5₃₃-4₄₀ transition near 475 GHz was clearly detected toward VY CMa and W Hya. Many other H₂O lines were detected toward all three target stars. Relative line intensity ratios and velocity widths were found to vary significantly from star to star. Maser action is observed in all but one line for which it was theoretically predicted. In contrast, one of the strongest maser lines, in R Leo by far the strongest, the 437GHz 7₅₃-6₆₀ transition, is not predicted to be inverted. Some other qualitative predictions of the model calculations are at variance with our observations. Plausible reasons for this are discussed. Based on our findings for W Hya and VY CMa, we find evidence that the H₂O masers in the AGB star W Hya arise from the regular circumstellar outflow, while shock excitation in a high velocity flow seems to be required to excite masers far from the red supergiant VY CMa.