An ALMA 3mm continuum census of Westerlund 1

TL;DR

This study used ALMA 3mm observations to measure mass-loss rates of massive stars in Westerlund 1, revealing how these rates vary with stellar evolution, the impact of binarity, and the influence of the environment on stellar winds and supernovae.

Contribution

First detailed ALMA 3mm continuum survey of Westerlund 1 providing mass-loss rates across diverse post-main sequence massive stars.

Findings

Mass-loss rates increase from OB supergiants to Wolf-Rayet stars.

Binarity significantly influences mass-loss rates, exemplified by Wd1-9.

Presence of compact nebulae suggests wind confinement by intra-cluster medium.

Abstract

Massive stars play an important role in both cluster and galactic evolution and the rate at which they lose mass is a key driver of both their own evolution and their interaction with the environment up to and including their SNe explosions. Young massive clusters provide an ideal opportunity to study a co-eval population of massive stars. We performed 3mm continuum observations with the Atacama Large Millimetre/submillimetre Array of the Galactic cluster Westerlund 1, to study the constituent massive stars and determine mass-loss rates for the diverse post-main sequence population. We detected emission from 50 stars in Westerlund 1, comprising all 21 Wolf-Rayets within the field of view, eight cool and 21 OB super-/hypergiants. Emission nebulae were associated with a number of the cool hypergiants while, unexpectedly, a number of hot stars also appear spatially resolved. We measured the mass-loss rates for a unique population of massive post-main sequence stars at every stage of evolution, confirming a significant increase as stars transition from OB supergiant to WR states. The range of spectral types exhibited provides a critical test of radiatively driven wind theory and the reality of the bi-stability jump. The extreme mass-loss rate inferred for the interacting binary Wd1-9 in comparison to other cluster members confirmed the key role binarity plays in massive stellar evolution. The presence of compact nebulae around a number of OB and WR stars is unexpected; by analogy to the cool super-/hypergiants we attribute this to confinement and sculpting of the stellar wind via interaction with the intra-cluster medium/wind. Given the morphology of core collapse SNe depend on the nature of the pre-explosion circumstellar environment, if this hypothesis is correct then the properties of the explosion depend not just on the progenitor, but also the environment in which it is located.