Wolf-Rayet spectra: how to tell binaries from singles

W.-R. Hamann and G. Gräfener

Universität Potsdam, Institut für Physik, Astrophysik

Many Wolf-Rayet stars are suspected to be binaries because their spectra are considered as looking composite. Corresponding criteria are the weakness of the lines ("diluted emission lines, d.e.l."), or the presence of absorption features. However, single WR stars can provide an amazing variety of spectra, depending on their parameters. Many of the "composite" spectra can be reproduced with single-star models. Synthetic spectra from the Potsdam Wolf-Rayet (PoWR) code have been employed to test the Galactic WN stars. It turns out that neither the weakness of emission lines (d.e.l.) nor the presence of absorption features give unique evidence for binarity. However, the spectral energy distribution over a wide range (UV, optical, IR) should reveal the spectral contamination from a companion. We checked eight d.e.l. suspects and 29 further WN stars, but found significant UV/optical contamination only in one case of a well-known binary. Thus the fraction of Galactic WN binaries might have been over-estimated so far.

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