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

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 similar to 35 confirmed members of Wd1 and similar to 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 angstrom 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 similar to 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 similar to 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 similar to 40%, a figure that is likely to increase as further data become available.