A VLT/FLAMES survey for massive binaries in Westerlund 1^*

I. First observations of luminous evolved stars

¹ Department of Physics and Astronomy, The Open University, Walton Hall, Milton Keynes MK7 6AA, UK e-mail: b.ritchie@open.ac.uk
² IBM United Kingdom Laboratories, Hursley Park, Winchester, Hampshire SO21 2JN, UK
³ Departamento de Física, Ingeniería de Sistemas y Teoría de la Señal, Universidad de Alicante, Apdo. 99, 03080 Alicante, Spain
⁴ Department of Physics and Astronomy, University of Sheffield, Sheffield S3 7RH, UK

Received: 12 June 2009
Accepted: 17 September 2009

Abstract

Aims. 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 the detection of 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, multi-epoch radial velocity survey that will provide the first direct constraints on these parameters.

Methods. VLT/FLAMES+GIRAFFE observations of Wd1 were made on seven epochs from late-June to early-September 2008, covering ~35 confirmed members of Wd1 and ~70 photometrically-selected candidate members. Each target was observed on a minimum of three epochs, with brighter cluster members observed on five (or, in a few cases, seven) occasions. Individual spectra cover the 8484–9001 Å range, and strong Paschen-series absorption lines are used to measure radial velocity changes in order to identify candidate binary systems for follow-up study.

Results. This study presents first-epoch results from twenty of the most luminous supergiant stars in Wd1. Four new OB supergiant members of Wd1 are identified, while statistically significant radial velocity changes are detected in ~60% of the 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 significant radial velocity changes of ~15–25 km s^-1 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%, a figure that is likely to increase as further data become available.