The Main Sequence of three Red Supergiant Clusters
  Massive clusters in our Galaxy are an ideal testbed to investigate the
properties and evolution of high mass stars. They provide statistically
significant samples of massive stars of uniform ages. To accurately determine
the intrinsic physical properties of these stars we need to establish the
distances, ages and reddening of the clusters. One avenue to achieve this is
the identification and characterisation of the main sequence members of red
supergiant rich clusters.
  Here we utilise publicly available data from the UKIDSS galactic plane
survey. We show that point spread function photometry in conjunction with
standard photometric decontamination techniques allows us to identify the most
likely main sequence members in the 10-20Myr old clusters RSGC1, 2, and 3. We
confirm the previous detection of the main sequence in RSGC2 and provide the
first main sequence detection in RSGC1 and RSGC3. There are in excess of 100
stars with more than 8Msun identified in each cluster. These main sequence
members are concentrated towards the spectroscopically confirmed red supergiant
stars. We utilise the J-K colours of the bright main sequence stars to
determine the K-band extinction towards the clusters. The differential
reddening is three times as large in the youngest cluster RSGC1 compared to the
two older clusters RSGC2 and RSGC3. Spectroscopic follow up of the cluster main
sequence stars should lead to more precise distance and age estimates for these
clusters as well as the determination of the stellar mass function in these
high mass environments.
