2008AJ....136..221F

We present K-band spectra of newly born OB stars in the obscured Galactic giant H II region W51A and ~0.8' angular resolution images in the J, H, and K_S-bands. Four objects have been spectroscopically classified as O-type stars. The mean spectroscopic parallax of the four stars gives a distance of 2.0±0.3 kpc (error in the mean), significantly smaller than the radio recombination line kinematic value of 5.5 kpc or the values derived from maser proper motion observations (6-8 kpc). The number of Lyman continuum photons from the contribution of all massive stars (NLyc ~1.5x10⁵⁰/s) is in good agreement with that inferred from radio recombination lines (NLyc = 1.3 x10⁵⁰/s) after accounting for the smaller distance derived here. We present analysis of archival high angular resolution images (NAOS CONICA at VLT and T-ReCS at Gemini) of the compact region W51 IRS 2. The K_S-band images resolve the infrared source IRS 2 indicating that it is a very young compact H II region. Sources IRS 2E was resolved into compact cluster (within 660 AU of projected distance) of three objects, but one of them is just bright extended emission. W51d1 and W51d2 were identified with compact clusters of three objects (maybe four in the case of W51d1) each one. Although IRS 2E is the brightest source in the K-band and at 12.6 µm, it is not clearly associated with a radio continuum source. Our spectrum of IRS 2E shows, similar to previous work, strong emission in Brγ and He I, as well as three forbidden emission lines of Fe III and emission lines of molecular hydrogen (H₂) marking it as a massive young stellar object.