Quantitative Near‐Infrared Spectroscopy of Of and WNL Stars

From new high signal-to-noise ratio (S/N) 1-2.2 μm spectroscopy of nine extreme early-type stars—including O Iaf, O Iafpe and WN9 types—we determine stellar parameters from detailed atmospheric analysis and evaluate results from near-IR analogues of well-known spectral diagnostics in the optical. We conclude that accurate stellar parameters can be measured from near-IR spectroscopy alone, an analysis technique important to studies of luminous stars in the Galactic center and other galaxies. Derived stellar parameters—mass-loss rates, luminosities, surface abundances, temperatures—show good agreement between optical and near-IR analyses, provided that IR data are of sufficient spectral resolution ( R >2000) and S/N (S/N>30). Wind velocities derived from He I 1.0830 μm are consistent with those from ultraviolet P Cygni profiles. Temperatures 200-1300 K systematically lower are determined from the near-IR diagnostics, a difference not significant in determining the stellar properties of these objects; which set of spectral lines provides the more accurate physical parameters—optical or IR—cannot at present be ascertained. The strength of He I 2.0581 μm is very sensitive to the extreme ultraviolet energy distribution where line blanketing by heavy elements plays an important role; this line should not on its own be considered a reliable temperature diagnostic. The three peculiar, extreme emission-line stars—the O Iafpe stars HD 152386, HD 152408, and HDE 313846—are more similar in both morphological and physical characteristics to WNL-type Wolf-Rayet stars than to normal O Iaf supergiants and should be classified as W-R. Their classification should be WN9ha, in which they remain a unique subgroup.