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arXiv · 2305.12779

Revisiting the orbital motion of WR 138

Gregor Rauw·Yaël Nazé·Eric Gosset
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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 $m_{\rm WN6o}/m_{\rm OB} = 0.53 \pm 0.09$. 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.

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Gregor Rauw, Yaël Nazé, Eric Gosset. 2023-05-22. Revisiting the orbital motion of WR 138. https://doi.org/10.1016/j.newast.2023.102062

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