Maser Observations of Westerlund 1 and Comprehensive Considerations on Maser Properties of Red Supergiants Associated with Massive Clusters

We report the results of Australia Telescope Compact Array (ATCA) observations of the Westerlund~1 (Wd1) region in the SiO v=1, J=1-0 and H2O 6(16)-5(23) maser lines, and we also report the analysis of maser properties of red supergiants (RSGs) associated with 6 massive clusters including Wd1. The primary purpose of this research is to explore possibilities of using maser emission for investigating the nature of massive clusters and associated RSGs. The SiO v=1, J=1-0 and H2O 6(16)-5(23) maser lines are detected toward 2 of 4 known RSGs in Wd1. The large velocity ranges of maser emission are consistent with the RSG status. RSGs with maser emission tend to exhibit redder log (F21/F12) and [K-12.13] colors compared to RSGs with no maser emission. The mass-loss rates derived from dust radiative transfer modeling suggest that RSGs with maser emission tend to exhibit larger mass-loss rates compared to RSGs with no maser emission. In an extended sample of 57 RSGs in 6 massive clusters, detections in the SiO line tend to homogeneously distribute in absolute luminosity L, whereas those in the H2O line tend to distribute in a region with large L values.