1ST DETECTION OF IONIZED HELIUM ABSORPTION-LINES IN INFRARED K-BAND SPECTRA OF O-TYPE STARS

We have obtained high signal-to-noise, moderate-resolution K band spectra of two early O-type main-sequence stars, HD 46150 05 V, and HD 46223 04 V, in the Rosette Nebula. We report the detection, for the first time, of the 2.189 mum He II line in O-type stars. Also detected is the 2.1661 mum Brgamma line in absorption. The 2.058 mum He I line appears to be present in absorption in both stars, although its appearance at our resolution is complicated by atmospheric features. These three lines can form the basis for a spectral classification system for hot stars in the K band that may be used at infrared wavelengths to elucidate the nature of those luminous stars in otherwise obscured H II and giant H II regions.