2011A&A...535A..40R

The distance of the very young open cluster Westerlund2, which contains the very massive binary system WR20a and is likely associated with a TeV source, has been the subject of much debate. We attempt a joint analysis of spectroscopic and photometric data of eclipsing binaries in the cluster to constrain its distance.A sample of 15 stars, including three eclipsing binaries (MSP44, MSP96, and MSP223) was monitored with the FLAMES multi-object spectrograph. The spectroscopic data are analysed together with existing BV photometry.The analysis of the three eclipsing binaries clearly supports the larger values of the distance, around 8kpc, and rules out values of about 2.4-2.8kpc that have been suggested in the literature. Furthermore, our spectroscopic monitoring reveals no clear signature of binarity with periods shorter than 50days in either the WN6ha star WR20b, the early O-type stars MSP18, MSP171, MSP182, MSP183, MSP199, and MSP203, or three previously unknown mid O-type stars. The only newly identified candidate binary system is MSP167. The absence of a binary signature is especially surprising for WR20b and MSP18, which were previously found to be bright X-ray sources. The distance of Westerlund2 is confirmed to be around 8kpc as previously suggested based on the spectrophotometry of its population of O-type stars and the analysis of the light curve of WR20a. Our results suggest that short-period binary systems are not likely to be common, at least not among the population of O-type stars in the cluster.