Palomar/TripleSpec observations of {\it Spitzer}/MIPSGAL~24\mic\
  circumstellar shells: unveiling the nature of their central sources
  We present near-IR spectroscopic observations of the central sources in 17
circumstellar shells from a sample of more than 400 "bubbles" discovered in the
Spitzer/MIPSGAL 24um survey of the Galactic plane and in the Cyg-X region. To
identify these shells, we have obtained J, H, and K band spectra with a
resolution ~2600 of the stars at their centers. We observed 14 MIPSGAL bubbles
(MBs), WR149, and 2 objects in the Cyg-X region (WR138a and BD+43 3710), our
sample being about 2.5 mag fainter in K band than previous studies of the
central sources of MBs. We use spectroscopic diagnostics and spectral libraries
to constrain the natures of our targets. We find five late type giants. The
equivalent widths of their CO 2.29um features allow us to determine their
spectral types and hence derive extinction along the line of sight, distance,
and physical size of the shells. We also find twelve early type stars, in nine
MBs and the 3 comparison objects. We find that the subtype inferred from the
near-IR for WR138a (WN9h) and WR149 (WN5h) agrees with that derived from
optical observations. A careful analysis of the literature and the environment
of BD+43 3710 allows us to rule out the carbon star nature previously
suggested. Our spectrum suggests that it is a B5I star. At the centers of the
nine MBs, we find a candidate [WC5-6], a candidate O5-6V star, a B0 supergiant,
a B/A type giant, and five LBV candidates. We report the detections of emission
lines arising from two shells with typical extents (~10") in agreement with
those in the mid-IR. We summarize the findings on the natures of the MBs since
their discovery, with 30% of them now known. Most MBs with central sources
detected in the near- to mid-IR have been identified and are red and blue
(super)giants, or stars evolving toward these phases including a handful of
newly discovered Wolf-Rayet stars and a significant number of LBV candidates.
