The Stellar Content of Obscured Galactic Giant HII Regions. VI: W51A

Abstract

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 \pm 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 propermotion observations (6--8 kpc). The number of Lyman continuum photons from the contribution of all massive stars (NLyc ~ 1.5 x 10^{50} s^{-1}) is in good agreement with that inferred from radio recombination lines (NLyc = 1.3 x 10^{50} s^{-1}) 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 IRS2. The K_S--band images resolve the infrared source IRS~2 indicating that it is a very young compact HII region. Sources IRS2E was resolved into compact cluster (within 660 AU of projected distance) of 3 objects, but one of them is just bright extended emission. W51d1 and W51d2 were identified with compact clusters of 3 objects (maybe 4 in the case of W51d1) each one. Although IRS~2E is the brightest source in the K-band and at 12.6 \micron, it is not clearly associated with a radio continuum source. Our spectrum of IRS~2E shows, similar to previous work, strong emission in Br$\gamma$ and HeI, as well as three forbidden emission lines of FeIII and emission lines of molecular hydrogen (H_2) marking it as a massive young stellar object.

arXiv comment31 pages and 9 figures, submitted to AJ

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• Spectrophotometric Distances to Galactic H\,{\sc{ii}} Regions
• Toward gas exhaustion in the W51 high-mass protoclusters
• A Multiwavelength Look at Galactic Massive Star Forming Regions
• A ring/disk/outflow system associated with W51 North: a very massive star in the making
• The P Cygni supergiant [OMN2000] LS1 - implications for the star formation history of W51
• On the Gaia DR2 distances for Galactic Luminous Blue Variables
• The spiral structure of the Galaxy revealed by CS sources and evidence for the 4:1 resonance
• Embedded Young Stellar Object Candidates in the Active Star Forming Complex W51: Mass Function and Spatial Distribution
• Young Stellar Objects in the Massive Star-Forming Regions W51 and W43
• A CO Line and Infrared Continuum Study of the Active Star-Forming Complex W51
• The Stellar Content of Obscured Galactic Giant HII Regions. VII. W3
• Mid Infrared View of the High Mass Star Formation Region W51A
• Near-Infrared Spectroscopy of Ultracompact \ion{H}{2} regions in W51A with NIFS/ALTAIR