Near-infrared spectra of Galactic stellar clusters detected on
  Spitzer/GLIMPSE images
  We present near-infrared spectroscopic observations of massive stars in three
stellar clusters located in the direction of the inner Galaxy. One of them, the
Quartet, is a new discovery while the other two were previously reported as
candidate clusters identified on mid-infrared Spitzer images (GLIMPSE20 and
GLIMPSE13). Using medium-resolution (R=900-1320) H and K spectroscopy, we
firmly establish the nature of the brightest stars in these clusters, yielding
new identifications of an early WC and two Ofpe/WN9 stars in the Quartet and an
early WC star in GLIMPSE20. We combine this information with the available
photometric measurements from 2MASS, to estimate cluster masses, ages, and
distances. The presence of several massive stars places the Quartet and
GLIMPSE20 among the small sample of known Galactic stellar clusters with masses
of a few 10^3 Msun, and ages from 3 to 8 Myr. We estimate a distance of about
3.5 kpc for Glimpse 20, and 6.0 kpc for Quartet. The large number of giant
stars identified in GLIMPSE13 indicates that it is another massive (~ 6500
Msun) cluster, but older, with an age between 30 and 100 Myr, at a distance of
about 3 kpc.
