The stellar content of obscured galactic giant HII regions. III. W31

We present near-infrared (J, H, and K) photometry and moderate-resolution (lambda/Delta lambda =3000) K-band spectroscopy of the embedded stellar cluster in the giant H II region W31. Four of the brightest five cluster members are early O-type stars based on their spectra. We derive a spectrophotometric distance for W31 of 3.4 +/-0.3 kpc using these new spectral types and infrared photometry. The brightest cluster source at K is a red object that lies in the region of the J-H versus H-K color-color plot inhabited by stars with excess emission in the K band. This point source has an H- plus K-band spectrum that shows no photospheric features, which we interpret as being the result of veiling by local dust emission. Strong Brackett series emission and permitted Fe II emission are detected in this source; the latter feature is suggestive of a dense inflow or outflow. The near-infrared position of this red source is consistent with the position of a 5 GHz thermal radio source seen in previous high angular resolution VLA images. We also identify several other K-band sources containing excess emission with compact radio sources. These objects may represent stars in the W31 cluster still embedded in their birth cocoons.