Revisiting the orbital motion of WR 138

Abstract

The optical spectrum of WR 138 exhibits emission lines typical of a WN6o star and absorption lines from a rapidly-rotating OB star. Using a large set of spectroscopic data, we establish a new orbital solution of the WN6o star based on the radial velocities of highly-ionized nitrogen lines. We show that the WN6o star moves on a 4.3 yr orbit with a comparatively low eccentricity of 0.16. The radial velocities of the OB star display considerable scatter. Our best estimates of the velocities of He I absorption lines result in a mass-ratio of $m_{WN6o}/m_{\mathrm{OB}}=0.53\pm 0.09$. We disentangle the spectra of the two stars and derive a projected rotational velocity of $v\sin i=350\pm 10$ km s${}^{-1}$ for the OB star. Our best orbital parameters, combined with the Gaia parallax of WR 138, are at odds with a previous interferometric detection of the companion, suggesting that there is either a bias in this detection or that WR 138 is actually a triple system.

arXiv commentaccepted for publication by New Astronomy

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