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Title:
The Stellar Content of Obscured Galactic Giant H II Regions. Vi. W51A
Authors:
Figuerêdo, E.; Blum, R. D.; Damineli, A.; Conti, P. S.; Barbosa, C. L.
Affiliation:
AA(Department of Physics and Astronomy, The Open University, Milton Keynes, MK7 6AA, UK ; Visiting Astronomer, Cerro Tololo Inter-American Observatory, National Optical Astronomy Observatory, which is operated by Associated Universities for Research in Astronomy, Inc., under cooperative agreement with the National Science Foundation.; ), AB(National Optical Astronomy Observatory, 950 North Cherry Avenue, Tucson, AZ, 85719, USA ), AC(IAG-USP, R. do Matão 1226, 05508-900, São Paulo, Brazil ; Visiting Astronomer, Cerro Tololo Inter-American Observatory, National Optical Astronomy Observatory, which is operated by Associated Universities for Research in Astronomy, Inc., under cooperative agreement with the National Science Foundation.; ), AD(JILA, University of Colorado, Campus Box 440, Boulder, CO 80309, USA ), AE(IP&D, Universidade do Vale do Paraíba, Av. Shihima Hifumi 2911, São José dos Campos 12244-000, SP, Brazil )
Publication:
The Astronomical Journal, Volume 136, Issue 1, pp. 221-233 (2008). (AJ Homepage)
Publication Date:
07/2008
Origin:
IOP
AJ Keywords:
H II regions, infrared: stars, stars: early-type, stars: formation, stars: fundamental parameters
DOI:
10.1088/0004-6256/136/1/221
Bibliographic Code:
2008AJ....136..221F

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 KS -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 ≈1.5 × 1050 s-1) is in good agreement with that inferred from radio recombination lines (NLyc = 1.3 ×1050 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 IRS 2. The KS -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.

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