Letter to the Editor

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

¹ Max-Planck-Institut für Kernphysik, Saupfercheckweg 1, D-69117 Heidelberg, Germany
² Zentrum für Astronomie der Universität Heidelberg, Astronomisches Rechen-Institut, Mönchhofstr. 12-14, 69120 Heidelberg, Germany
³ Max-Planck-Institut für Astronomie, Königstuhl 17, D-69117 Heidelberg, Germany
⁴ Astronomy & Astrophysics Section, School of Cosmic Physics, Dublin Institute for Advanced Studies, DIAS Dunsink Observatory, Dublin D15 XR2R, Ireland
⁵ Max Planck Institute for Astrophysics, Karl-Schwarzschild-Str. 1, 85748 Garching, Germany
⁶ Universität Heidelberg, Interdisziplinäres Zentrum für Wissenschaftliches Rechnen, 69120 Heidelberg, Germany
⁷ Istituto Nazionale di Astrofisica (INAF) – Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy
⁸ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany
⁹ Lockheed Martin Solar and Astrophysics Laboratory, 3251 Hanover Street, Palo Alto, CA 94304, USA
¹⁰ School of Physical and Chemical Sciences, Queen Mary University of London, Mile End, London E1 4NS, UK
¹¹ Gemini Observatory/NSFs NOIRLab, 950 N. Cherry Ave., Tucson, AZ 85719, USA
¹² Departamento de Astrofísica, Centro de Astrobiología, CSIC-INTA, Ctra. Torrejón a Ajalvir km 4, E-28850 Torrejón de Ardoz, Spain
¹³ Department of Physics and Astronomy, The Open University, Walton Hall, Milton Keynes MK7 6AA, UK

^⋆ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 29 January 2026
Accepted: 20 March 2026

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.