AK‐Band Spectral Atlas of Wolf‐Rayet Stars

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Abstract
We present K-band spectra (R ~ 525) of 38 northern Galactic WR Stars, of which 16 are WC, 19 are WN, two are WN/WC, and one is WO. The spectra have the expected trend of stronger lines for higher ionization species with earlier spectral subtype. Spectra for the late WC stars can appear to have weak emission lines, an effect due to different amounts of dust dilution in the individual stars. There are also differences in spectral morphology for stars within other subtypes. In general, the spectra for all WC stars earlier than WC9 tend to be quite similar, while the spectra for WN subtypes are more easily differentiated. Several previously unidentified emission lines are seen in the spectra, most notably, features near 2.247 and 2.368 μm in late-type WN stars, and one near 2.222 μm in late-type WC stars. We attribute the 2.247 μm line to a N III transition and argue that it might provide the best method for discriminating between WNL and OIf+ stars in the K band. We investigate the behavior of the 2.11 μm (He I + N III), 2.166 μm (H I + He I + He II), and 2.189 μm (He II) emission lines in WN types and find that these lines provide for accurate discrimination within the sample to within 1 subtype. From this investigation, it appears that the ratio of W2.189 μm/W2.11 μm is sensitive to subtype and shows the least dispersion within subtypes. In addition, we find that the W2.189 μm/W2.166 μm ratio also scales with subtype in a well-behaved manner once it is corrected for contamination of the 2.166 μm line by He II 14-8 and for the presence of an O star companion in binary systems. We also investigate the behavior of the 2.058 μm (He I), 2.08 μm (C IV), and the 2.11 μm (C III + He I) emission lines in WC types. The ratio of W2.08 μm/W2.11 μm correlates with subtype; however, it is not easy to distinguish between individual subtypes earlier than WC8 by just using this quantity. The dominance of the 2.058 μm line in WC9 types distinguishes this subtype from all other WC subtypes. Two WC9 stars in our sample have nearly featureless spectra due to dust dilution. It is possible to classify a WR star to within one subtype in the WN sequence based upon the sample in this atlas. The similarity of WC spectra makes it difficult to distinguish among individual subtypes earlier than WC8.