Other
Scientific paper
Jan 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995phdt........22h&link_type=abstract
Thesis (PH.D.)--UNIVERSITY OF COLORADO AT BOULDER, 1995.Source: Dissertation Abstracts International, Volume: 57-02, Section: B,
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3
Scientific paper
In order to learn about the early formation and evolution of massive stars, one must look towards their birthplaces. Here, behind tens of magnitudes of visible extinction, the young stars can be found still forming. At wavelengths of a few microns, radiation from these stars can more easily escape their shrouded environment. With the recent advent of near-infrared detectors, such stars can now be studied directly. I have developed a spectroscopic classification system for O and B-type stars in the near-infrared atmospheric window from 2.0 to 2.2 mum. To do so, I obtained high signal-to-noise, moderate resolution 2 μm (K-band) spectra of 130 optically visible massive stars. A majority of the spectra are of O- and B-type stars; however, I have extended this study to cool, very luminous stars, which represent a more advanced evolutionary stage of massive stars. I report the detection, for the first time, of the 2.189 μm He II absorption line in O-type stars. Also detected in nearly all hot stars is the 2.1661 μm H I (Br gamma) line. Other useful lines in my spectroscopic classification system include the 2.058 μm singlet and the 2.1127 μm triplet He I lines. Two sets of metal lines are present in the hottest stars, the C IV triplet at 2.07 μm and a N III line at 2.115 mum. The 2 μm spectroscopic signatures of O- and B-type stars are rather well behaved, showing smooth variations with temperature and luminosity. The 2.0-2.2 μm region can be used to elucidate the nature of hot, luminous stars in otherwise optically obscured star forming regions. I have obtained high-quality 2 mu m spectra of more than 30 stars in the direction of the very young, heavily extinguished Galactic star-forming region M17. I have identified seven stars as O type from their spectral lines of H I, He I, He II, and N III using the 2 μm spectral classification system that I developed for this purpose. Two of these O stars are behind more than 15 magnitudes of visible extinction and provide unprecedented opportunities for absorption studies of dark interstellar clouds at optical and near -infrared wavelengths. The O stars found are able to provide the number of Lyman continuum photons required to explain radio continuum observations of the region. A set of stellar objects, all but one with strong excess emission in the infrared, show completely different spectral characteristics from known main-sequence stars. Three are completely featureless throughout the 2 μm window, four (possibly five) show molecular CO in emission and two have CO in absorption combined with extremely large near-infrared excesses. Extrapolating from the number of early-O stars found in the M17 region and assuming a normal stellar mass function, photometric surveys of the field suggest the number of late-O and B stars found is far below the number expected. The peculiar stellar objects I found may be massive, young stellar objects, possibly the "missing" late-O and B stars, still shrouded by circumstellar material. Because the early-O stars in the field are already free of their circumstellar material, the most massive stars must have a very short accretion phase. (Abstract shortened by UMI.).
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