A VLT/FLAMES survey for massive binaries in Westerlund 1: VIII. Binary Systems and Orbital Parameters

TL;DR

This study uses multi-epoch spectroscopy to identify and characterize binary systems among massive stars in Westerlund 1, significantly increasing the known binary population and providing new orbital parameters.

Contribution

It presents the first long-period binary in Westerlund 1 and refines the binary fraction among massive stars using extensive spectroscopic data.

Findings

Identified 20 new OB binaries and several Wolf-Rayet binaries.

First detection of a long-period binary in Westerlund 1.

Estimated binary fraction of at least 40% among OB stars.

Abstract

The galactic cluster Westerlund 1 contains a rich population of evolved, massive stars, and a high binary fraction has been inferred from previous multiwavelength observations. We use multi-epoch spectroscopy of a large sample of early-type stars to identify new binaries and binary candidates in the cluster. VLT/FLAMES was used to obtain spectra of ~100 OB stars over a 14-month baseline in 2008 and 2009, supplemented with follow-up observations in 2011 and 2013, and we identify 20 new OB I--III binaries, a WN9h binary, and a WC9d binary, greatly increasing the number of directly confirmed binary systems in Westerlund 1, while 12 O9--9.5 Iab--III stars are identified as candidate binaries. The 173.9 day SB1 W1030 represents the first longer-period system identified in the cluster, while the determination of a 53.95 day period for W44/L makes it the first Wolf-Rayet binary in Westerlund 1 with a confirmed orbital period greater than ten days. Our results suggest the binary fraction in the OB population is at least 40%, and may be significantly higher. (Abridged)