A VLT/FLAMES survey for massive binaries in Westerlund 1: I. first observations of luminous evolved stars

TL;DR

This study conducts a multi-epoch radial velocity survey of luminous supergiants in Westerlund 1 to directly measure binary properties, revealing a high binary fraction and identifying several probable short-period binaries.

Contribution

It provides the first direct constraints on binary parameters in Westerlund 1 using multi-epoch spectroscopy, improving understanding of massive star binarity in clusters.

Findings

Approximately 60% of targets show significant radial velocity changes.

Identification of W43a as a short-period binary.

Binary fraction among supergiants exceeds 40%.

Abstract

Multiwavelength observations of the young massive cluster Westerlund 1 have revealed evidence for a large number of OB supergiant and Wolf-Rayet binaries. However, in most cases these findings are based on secondary binary characteristics such as hard X-ray emission and/or non-thermal radio spectra and hence provide little information on binary properties such as mass ratio and orbital period. To overcome this shortcoming we have initiated a long temporal baseline, VLT/FLAMES+GIRAFFE multi-epoch radial velocity survey that will provide the first direct constraints on these parameters. This study presents first results from twenty of the most luminous supergiant stars in Wd1. Statistically significant radial velocity changes are detected in ~60% of targets. W43a is identified as a short-period binary, while W234 and the newly-identified cluster member W3003 are probable binaries and W2a is a strong binary candidate. The cool hypergiants W243 and W265 display photospheric pulsations, while a number of early-mid B supergiants display radial velocity changes that we cannot distinguish between orbital or photospheric motion in our initial short-baseline survey. When combined with existing observations, we find 30% of our sample to be binary (6/20) while additional candidate binaries support a binary fraction amongst Wd1 supergiants in excess of ~40% [ABRIDGED]