A SPECTRAL ATLAS OF HOT, LUMINOUS STARS AT 2 MICRONS

We present 2 mu m (K band) spectra of 180 well-studied, optically visi ble, 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, co ol hypergiant stars, and high-mass X-ray binary stars. Our aim in stud ying normal OB stars is to develop an empirical relationship between 2 mu m spectral features of these massive stars and their stellar tempe rature and luminosity. We find the system of lines between 2.0 and 2.2 mu m is particularly good for differentiating the early- and mid-O ty pe stars. In the late-O and early-B stars, differentiation becomes mor e difficult, as the features show only moderate changes. We have devel oped a spectral classification system for the K band to be used to est imate effective temperatures of O and early-B stars. We demonstrate th at K-band spectroscopy is superior in estimating the temperature of ho t, luminous stars than the traditional methods of using infrared or ev en optical photometric colors alone. The only requirements are that ad equate resolution (R > 1000) and signal-to-noise (S/N similar to 70) b e achieved. With our classification system, stars behind large amounts of visible extinction, such as in young, heavily reddened H II region s throughout our Galaxy, may be identified and studied for the first t ime through 2 mu m spectroscopy. Emission lines are commonly seen in t he K-band spectra of supergiant stars, however, the OBN supergiants, w hich 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 mu m. This has led us to propose an evolutionary scenario for some of the Galactic center He I emission-line stars, whi ch evokes rotational mixing (Maeder 1987; Langer 1992) to explain both the strong line emission and high luminosity of these mysterious sour ces. 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 mu m of Schaerer et al. (1996b) and thos e observed in OB stars. Finally, we include a thorough discussion of t he 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.