Spectral classification of the brightest objects in the galactic star
  forming region W40
  We present high S/N, moderate resolution near-infrared spectra, as well as 10
micron imaging, for the brightest members of the central stellar cluster in the
W40 HII region, obtained using the SpeX and MIRSI instruments at NASA's
Infrared Telescope Facility. Using these observations combined with archival
Spitzer Space Telescope data, we have determined the spectral classifications,
extinction, distances, and spectral energy distributions for the brightest
members of the cluster. Of the eight objects observed, we identify four main
sequence (MS) OB stars, two Herbig Ae/Be stars, and two low-mass young stellar
objects. Strong HeI absorption at 1.083 micron in the MS star spectra strongly
suggests that at least some of these sources are in fact close binaries. Two
out of the four MS stars also show significant infrared excesses typical of
circumstellar disks. Extinctions and distances were determined for each MS star
by fitting model stellar atmospheres to the SEDs. We estimate a distance to the
cluster of between 455 and 535 pc, which agrees well with earlier (but far less
precise) distance estimates. We conclude that the late-O star we identify is
the dominant source of LyC luminosity needed to power the W40 HII region and is
the likely source of the stellar wind that has blown a large (~4 pc)
pinched-waist bubble observed in wide field mid-IR images. We also suggest that
3.6 cm radio emission observed from some of the sources in the cluster is
likely not due to emission from ultra-compact HII regions, as suggested in
other work, due to size constraints based on our derived distance to the
cluster. Finally, we also present a discussion of the curious source IRS 3A,
which has a very strong mid-IR excess (despite its B3 MS classification) and
appears to be embedded in a dusty envelope roughly 2700 AU in size.
