Visual Orbits of Wolf-Rayet Stars II: The Orbit of the Nitrogen-Rich WR Binary WR 138 measured with the CHARA Array

TL;DR

This study combines interferometric and spectroscopic data to precisely determine the three-dimensional orbit and stellar masses of the Wolf-Rayet binary WR 138, providing insights into its formation and evolution.

Contribution

It presents the first detailed 3D orbital parameters and stellar masses of WR 138 using combined interferometric and spectroscopic data, improving precision over previous methods.

Findings

Stellar masses: WR star 13.93±1.49 M☉, O star 26.28±1.71 M☉

Orbital parallax matches Gaia distance of 2.13 kpc

System likely formed with little interaction, possibly during red supergiant phase

Abstract

Classical Wolf-Rayet stars are descendants of massive OB-type stars that have lost their hydrogen-rich envelopes, and are in the final stages of stellar evolution, possibly exploding as type Ib/c supernovae. It is understood that the mechanisms driving this mass-loss are either strong stellar winds and or binary interactions, so intense studies of these binaries including their evolution can tell us about the importance of the two pathways in WR formation. WR 138 (HD 193077) has a period of just over 4 years and was previously reported to be resolved through interferometry. We report on new interferometric data combined with spectroscopic radial velocities in order to provide a three-dimensional orbit of the system. The precision on our parameters tend to be about an order of magnitude better than previous spectroscopic techniques. These measurements provide masses of the stars, namely $M_{\rm WR} = 13.93\pm1.49M_{\odot}$ and $M_{\rm O} = 26.28\pm1.71M_{\odot}$. The derived orbital parallax agrees with the parallax from \textit{Gaia}, namely with a distance of 2.13 kpc. We compare the system's orbit to models from BPASS, showing that the system likely may have been formed with little interaction but could have formed through some binary interactions either following or at the start of a red supergiant phase, but with the most likely scenario occurring as the red supergiant phase starts for a $\sim 40M_\odot$ star.