A multi-transition submillimeter water maser study of evolved stars - detection of a new line near 475 GHz

TL;DR

This study used the APEX telescope to observe multiple submillimeter water maser lines in evolved stars, discovering a new line near 475 GHz and analyzing maser excitation mechanisms and star-specific conditions.

Contribution

It reports the first detection of the 475 GHz water maser line in evolved stars and compares observational data with theoretical predictions, highlighting discrepancies and insights into maser excitation.

Findings

Detection of the 475 GHz water maser line in VY CMa and W Hya.

Variation in line intensity ratios and velocity widths among stars.

Evidence of different maser excitation mechanisms in AGB stars and supergiants.

Abstract

Context: 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. Aims: 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 475 GHz was conducted toward these stars. Methods: The Atacama Pathfinder Experiment telescope was used for a multi-transition observational study of submillimeter H2O lines. Results: The 5_33-4_40 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. Conclusions: 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 437 GHz 7_53-6_60 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.