# A VLT/FLAMES survey for massive binaries in Westerlund 1: VII. Cluster census

## TL;DR

This study provides a comprehensive census of massive stars in Westerlund 1, revealing the importance of binary interactions in their evolution and the diversity of stellar types present, which informs models of stellar evolution and core-collapse outcomes.

## Contribution

It offers the first extensive stellar census of Westerlund 1, highlighting the role of binarity in the formation of hypergiants and the diversity of massive star populations.

## Key findings

- 166 stars identified with initial masses 25-50Msun
- Binarity linked to the formation of hypergiants and Wolf-Rayets
- Diverse stellar evolutionary pathways observed

## Abstract

The formation, properties, and evolution of massive stars remain subject to considerable uncertainty; impacting on fields as diverse as galactic feedback and the nature of the progenitors of both electromagnetic and gravitational wave transients. The clusters many such stars reside within provide a unique laboratory for addressing these issues, and in this work we provide a comprehensive stellar census of Westerlund 1 to underpin such efforts. 69 new members were identified via I-band spectroscopy, yielding a total cluster population of 166 stars with initial masses of ~25Msun to ~50Msun, with more massive stars already lost to supernova. The stellar population follows a smooth and continuous morphological sequence from late-O giant through to OB supergiant. Subsequently, the progression bifurcates, with one branch yielding mid- to late-B hypergiants and cool super-/hypergiants, and the other massive blue stragglers, prior to a diverse population of H-depleted Wolf-Rayets. A substantial population of O-type stars with anomalously broad Paschen series lines are seen, a property which we attribute to binarity. Binary interaction is clearly required to yield the uniquely rich cohort of hypergiants, which includes both mass-stripped primaries and rejuvenated secondaries/stellar mergers. As a consequence future observations of Wd1 and similar stellar aggregates hold out the prospect of characterising both single- and binary- evolutionary channels for massive stars and determining their relative contributions. This in turn will permit the physical properties of such objects at the point of core-collapse to be predicted; of direct relevance for understanding the formation of relativistic remnants such as the magnetars associated with Wd1 and other young massive clusters (Abridged).