A spectral atlas of hot, luminous stars at 2 microns

We present 2 μm (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 μm spectral features of these massive stars and their stellar temperature and luminosity. We find the system of lines between 2.0 and 2.2 μm is particularly 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 in young, heavily reddened H II regions throughout our Galaxy, may be identified and studied for the first time through 2 μm spectroscopy. Emission lines are commonly seen in the K-band spectra of supergiant stars, however, the OBN super- giants, 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 μm. 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 μm 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.