The First Dynamical Mass Determination of a Nitrogen-rich Wolf-Rayet Star using a Combined Visual and Spectroscopic Orbit

TL;DR

This paper reports the first visual orbit and dynamical mass measurements for the nitrogen-rich Wolf-Rayet binary WR 133, combining interferometric, spectroscopic, and Gaia data to improve understanding of massive star evolution.

Contribution

It provides the first visual orbit for a WN star and combines multiple observational methods to derive stellar masses and orbital parameters.

Findings

Derived masses: WR star ~9.3 M_sun, O star ~22.6 M_sun.

First visual orbit for a WN star and third for a Wolf-Rayet star.

Masses are lower than expected from spectral types, suggesting binary interactions.

Abstract

We present the first visual orbit for the nitrogen-rich Wolf-Rayet binary, WR 133 (WN5o + O9I) based on observations made with the CHARA Array and the MIRC-X combiner. This orbit represents the first visual orbit for a WN star and only the third Wolf-Rayet star with a visual orbit. The orbit has a period of 112.8 d, a moderate eccentricity of 0.36, and a separation of $a$= 0.79 mas on the sky. We combine the visual orbit with an SB2 orbit and Gaia parallax to find that the derived masses of the component stars are $M_{\rm WR}$ = $9.3\pm1.6 M_\odot$ and $M_{\rm O}$ = $22.6\pm 3.2 M_\odot$, with the large errors owing to the nearly face-on geometry of the system combined with errors in the spectroscopic parameters. We also derive an orbital parallax that is identical to the {\it Gaia}-determined distance. We present a preliminary spectral analysis and atmosphere models of the component stars, and find the mass-loss rate in agreement with polarization variability and our orbit. However, the derived masses are low compared to the spectral types and spectral model. Given the close binary nature, we suspect that WR 133 should have formed through binary interactions, and represents an ideal target for testing evolutionary models given its membership in the cluster NGC 6871.