We present new high-resolution near-IR spectroscopy and OH maser observations to investigate the population of cool luminous stars of the young massive Galactic cluster RSGC1 .
Using the 2.293µm CO-bandhead feature , we make high-precision radial velocity measurements of 16 of the 17 candidate Red Supergiants ( RSGs ) identified by Figer et al .
We show that F16 and F17 are foreground stars , while we confirm that the rest are indeed physically-associated RSGs .
We determine that Star F15 , also associated with the cluster , is a Yellow Hypergiant based on its luminosity and spectroscopic similarity to \rho Cas .
Using the cluster ’ s radial velocity , we have derived the kinematic distance to the cluster and revisited the stars ’ temperatures and luminosities .
We find a larger spread of luminosities than in the discovery paper , consistent with a cluster age 30 % older than previously thought ( 12 \pm 2Myr ) , and a total initial mass of ( 3 \pm 1 ) \times 10 ^ { 4 } M _ { \normalsize \odot } .
The spatial coincidence of the OH maser with F13 , combined with similar radial velocities , is compelling evidence that the two are related .
Combining our results with recent SiO and H _ { 2 } O maser observations , we find that those stars with maser emission are the most luminous in the cluster .
From this we suggest that the maser-active phase is associated with the end of the RSG stage , when the luminosity-mass ratios are at their highest .