Submillimetre water masers at 437, 439, 471, and 474 GHz towards evolved stars. APEX observations and radiative transfer modelling

TL;DR

This study observes and models submillimetre water masers in evolved stars, revealing their common presence, likely origin in inner envelopes, and sensitivity to physical conditions, especially dust temperature, with implications for understanding stellar mass loss and shock processes.

Contribution

It provides the first comprehensive observational and radiative transfer analysis of 437-474 GHz water masers in evolved stars, linking maser presence to stellar variability and physical conditions.

Findings

7 out of 11 stars show water masers at studied frequencies.

Maser lines are detected near stellar velocities, indicating inner envelope origin.

Maser strength is highly sensitive to dust temperature and physical conditions.

Abstract

Here we aim to characterise submillimetre water masers at 437, 439, 471, and 474 GHz towards a sample of evolved stars. We used the Atacama Pathfinder Experiment (APEX) to observe submillimetre water transitions and the CO (4-3) line towards 11 evolved stars. The sample included semi-regular and Mira variables, plus a red supergiant star. We performed radiative transfer modelling for the water masers. We also used the CO observations to determine mass loss rates for the stars. From the sample of 11 evolved stars, 7 display one or more of the masers at 437, 439, 471, and 474 GHz. We therefore find that these masers are common in evolved star circumstellar envelopes. The fact that the maser lines are detected near the stellar velocity indicates that they are likely to originate from the inner circumstellar envelopes of our targets. We tentatively link the presence of masers to the degree of variability of the target star, that is, masers are more likely to be present in Mira variables than in semi-regular variables. We suggest that this indicates the importance of strong shocks in creating the necessary conditions for the masers. Typically, the 437 GHz line is the strongest maser line observed among those studied here. We cannot reproduce the above finding in our radiative transfer models. In general, we find that maser emission is very sensitive to dust temperature in the lines studied here. To produce strong maser emission, the dust temperature must be significantly lower than the gas kinetic temperature. In addition to running grids of models in order to determine the optimum physical conditions for strong masers in these lines, we performed smooth wind modelling for which we cannot reproduce the observed line shapes. This also suggests that the masers must originate predominantly from the inner envelopes.