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.

accepted for publication by New Astronomy

References in corpus (9)

• Binary interaction dominates the evolution of massive stars
• Estimating distances from parallaxes. V: Geometric and photogeometric distances to 1.47 billion stars in Gaia Early Data Release 3
• Physical Properties of Wolf-Rayet Stars
• Interacting Binaries with Eccentric Orbits. Secular Orbital Evolution Due To Conservative Mass Transfer
• Interacting Binaries with Eccentric Orbits II. Secular Orbital Evolution Due To Non-Conservative Mass Transfer
• Line Luminosities of Galactic and Magellanic Cloud Wolf-Rayet stars
• Red noise and pulsations in evolved massive stars
• A spectroscopic multiplicity survey of Galactic Wolf-Rayet stars: II. The northern WNE sequence
• The CHARA Array resolves the long-period Wolf-Rayet binaries WR 137 and WR 138