Maser and infrared properties of red supergiants embedded in the superstar cluster Westerlund 1

Abstract

The properties of astrophysical masers are governed by the physical conditions

of circumstellar envelopes of evolved stars, implying a connection between

maser and mass-loss phenomena. Although similar studies to another type of

mass-losing star, Asymptotic Giant Branch (AGB) stars, have been conducted,

a quantitative understanding for the case of Red Supergiants (RSGs) is still

largely lacking. Recently, more and more galactic massive clusters are discovered

and studied, which provide a coeval environment to investigate the evolution

of cool massive evolved stars with similar distance and metallicity. Among

them, Westerlund 1 (Wd1) is one of the most massive open clusters, classified

as Super Star Clusters (SSCs), within the Milky Way harboring 4 RSGs.

We have 2 goals in this project. We aim to investigate the relationship between

SiO maser detection and mass-loss behavior of RSGs in massive clusters,

in order to explore the possibility of developing SiO maser as an evolutionary

indicator of RSGs. We also aim to determine the kinematic properties of Wd1

with the aid of SiO maser.

We report on observation of 43 GHz SiO (v = 1, J = 1–0) maser towards

Wd1 conducted with Australia Telescope Compact Array (ATCA). We also

make use of archival 22 GHz H2O maser (616–523) in help of studying kinematic

of Wd1. We derive the maser detection information and stellar velocities from

maser data. We also compile an extra sample of 36 RSGs, which surveyed in SiO

maser before, within massive clusters, e.g. Red Supergiant Cluster 1 (RSGC1),

Red Supergiant Cluster 2 (RSGC2), Perseus OB association 1 (Per OB 1) and

Mercer et al.’s #8 (Mc8), and organize their photometric data from 2MASS,

GLIMPSE, MSX, AKARI and WISE to derive their spectral energy distributions

(SEDs). Luminosity, effective temperature and mass-loss rate are derived

from fitting the SEDs with DUSTY model in order to search for correlations

between mass-loss rate, SiO maser properties and evolution of RSGs.

We detected SiO maser emission towards 2 RSGs (W 237 and W 26) in

Wd1. These 2 RSGs also emit H2O maser as revealed from archival H2O maser

observation data. We noticed that SiO masers tend to be detected towards RSGs

with higher luminosity and mass-loss rate. We also determined the kinematic

distance of Wd1 to be 3.3 kpc. From that result, we conclude that SiO maser

is an effective evolution indicator for RSGs, which traces RSGs with high massloss

rate. We also show that the age of RSGs in Wd1 is 10 ± 2 Myr which

support the multi-generation star formation scenario in Wd1.