Previous abstract Next abstract

Session 110 -- Open Clusters
Display presentation, Saturday, January 15, 9:30-6:45, Salons I/II Room (Crystal Gateway)


M.M.Hanson, P.S.Conti (JILA, U.\ of Colorado)

We present 2 $\mu$m spectra of O and early-B main sequence stars. These stars show absorption features at Br~$\gamma$, 2.112 $\mu$m He~I, 2.189 $\mu$m He~II, and emission at 2.116$\mu$m N~III, which vary with spectral sub-type in a sufficiently ``well behaved'' manner that they can be used as a rough classification system for hot stars. We also show spectra of several O8f, late-O and early-B giant and supergiant stars. Subtype classification of these stars is also possible but the luminosity criteria are more subtle. We estimate that all together some 50 stellar spectra will be necessary to establish the system, which we are presently pursuing. Our final goal is to use the system to enumerate the stellar content of deeply enshrouded giant galactic HII regions, such as M17, NGC 3576 and W51. With 2 $\mu$m spectroscopy, we can study the very youngest of the most massive stars, before the combined effects of stellar radiation and winds clear away the dust to reveal the H~II regions and the star populations at optical wavelengths.

Our spectra are the first taken showing the helium and Br~$\gamma$ line behavior in a series of main sequence OB and luminous late-O, early-B-type giant and supergiant stars. The K band spectra of the Galactic Center (GC) ``He~I" stars (Krabbe et al. 1991, ApJ , 382 , L19) are unlike any of our ``standard'' stars. A number of the GC stars show both the Br~$\gamma$ and 2.058 $\mu$m He~I lines in emission. Such lines have been seen in peculiar, luminous, and highly evolved massive stars in the LMC, and thus the ``He~I'' stars are generically described as ``Luminous Blue Variables'' and WN9/Ofpe, though problems still exist with these identifications. The GC stars appear to be superficially similar to one of our program stars, the Oe star HD~155806 (i.e. emission at 2.058 $\mu$m He~I and Br~$\gamma$). Morphologically similar emission features are observed in the visible for this star and have been interpreted as due to a disk-like geometry viewed from a near polar direction. The emission line width in our Oe star is narrow and unresolved ($\Delta v < 300$ km s$^{-1}$), whereas several of the the GC stars have widths more like 1000 km s$^{-1}$.

Saturday program listing