EWOCS-V: Is Wd1-72 a recent post-interaction WR+O binary?

TL;DR

This paper investigates the origin of Wd1-72, a Wolf-Rayet binary, proposing it is a recent post-interaction binary formed through Roche Lobe Overflow, supported by hydrodynamic simulations matching observed nebulosity.

Contribution

It introduces a hydrodynamic model of non-conservative RLOF mass-loss explaining Wd1-72's morphology and suggests it is a recent post-interaction binary at about 10 kyr post-RLOF.

Findings

Hydrodynamic simulation reproduces nebular morphology around Wd1-72.

Wd1-72 likely experienced a second or subsequent RLOF episode.

W1-72 may be only ~10 kyr post-RLOF, indicating recent binary interaction.

Abstract

The evolutionary origin of Wolf-Rayet (WR) stars at Solar metallicity is unclear. Single-star evolution from massive O stars, possibly via a Luminous Blue Variable phase, is challenged by binary period distributions of different WR subtypes. Wd1-72 is a WN7b+O binary embedded in the collective wind of the Galactic young massive cluster Westerlund 1 (Wd 1). It is surrounded by highly structured nebulosity, with cometary tails pointing away from Wd 1 and quasi-spherical droplets towards it. In this letter, we demonstrate that this morphology can be qualitatively reproduced by a hydrodynamic simulation of non-conservative Roche Lobe Overflow (RLOF) mass-loss into a cluster wind. Our model is based on a detailed binary evolution track consistent with key known properties of Wd1-72. Our work suggests Wd1-72 could be only ~10 kyr post-RLOF, and the hydrogen-free nature of Wd1-72 favours this being a second or subsequent RLOF episode. Follow-up observations could make Wd1-72 a valuable benchmark for probing mass-loss and mass-transfer in forming gravitational-wave binary-progenitor systems.