THE ASTROPHYSICAL JOURNAL SUPPLEMENT SERIES, 107:281-311, 1996 November

             A SPECTRAL ATLAS OF HOT, LUMINOUS STARS AT 2 MICRONS

                         M. M. HANSON AND P. S. CONTI
      JILA Campus Box 440, University of Colorado, Boulder CO 80309-0440;
                            mhanson@as.arizona.edu

                                      AND

                                  M. J. RIEKE
        Steward Observatory, Department of Astronomy, Tucson, AZ 85721


                                   ABSTRACT

        We present 2 um (K band) spectra of 180 well-studied, optically
     visible, luminous stars.  Most of the stars are of OB spectral type,
     but we have also included a number of Oe and Be stars, OBN and OBC
     stars, cool hypergiant stars, and high-mass X-ray binary stars.  Our
     aim in studying normal OB stars is to develop an empirical
     relationship between 2 um spectral features of these massive stars and
     their stellar temperature and luminosity.  We find the system of lines
     between 2.0 and 2.2 um is particulary good for differentiating the
     early- and mid-O type stars.  In the late-O and early-B stars,
     differentiation becomes more difficult, as the features show only
     moderate changes.  We have developed a spectral classification system
     for the K band to be used to estimate effective temperatures of O and
     early-B stars.  We demonstrate that K-band spectroscopy is superior in
     estimating the temperature of hot, luminous stars than the traditional
     methods of using infrared or even optical photometric colors alone.
     The only requirements are that adequate resolution (R > 1000) and
     signal-to-noise (S/N ~ 70) be achieved.  With our classification
     system, stars behind large amounts of visible extinction, such as
     young, heavily reddened H II regions throughout our Galaxy, may be
     identified and studied for the first time through 2 um spectroscopy.
        Emission lines are commonly seen in the K-band spectra of supergiant
     stars, however, the OBN supergiants, which have a higher ratio of some
     processed materials at their surface, may be more likely to show line
     emission, especially the He I singlet transition at 2.058 um.  This
     has led us to propose an evolutionary scenario for some of the
     Galactic center He I emission-line stars, which evokes rotational
     mixing (Maeder 1987; Langer 1992) to explain both the strong line
     emission and high luminosity of these mysterious sources.
        We have compared our spectroscopic database with the most recent
     stellar atmosphere models.  We are encouraged by the good match
     between the model line profiles at 2 um of Schaerer et al. (1996b) and
     those observed in OB stars.  Finally, we include a thorough discussion
     of the observational and reduction methods employed to obtain the
     spectra shown in this atlas for the benefit of those wishing to obtain
     similar, classification-quality, near-infrared spectra.

     Subject headings: atlases -- infrared: stars -- stars: early-type --
                       stars: fundamental parameters