A VLT/FLAMES survey for massive binaries in Westerlund 1

Abstract

Context. The galactic cluster Westerlund 1 contains a rich population of evolved, massive stars. A high binary fraction has been inferred from previous multi-wavelength observations. Aims. We use multi-epoch spectroscopy of a large sample of early-type stars in Westerlund 1 to identify new binaries and binary candidates in the cluster. Methods. VLT/FLAMES was used with the GIRAFFE spectrograph in HR21 mode 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. Radial velocities were obtained from strong Paschen series absorption lines in the I-band. Results. 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 based on radial velocity changes that are inconsistent with photospheric variability. 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 (WR 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. Conclusions. These results demonstrate that binary systems can be effectively identified in the population of OB I−III stars evolving off the main sequence in Westerlund 1. Future multi-epoch surveys will be able to fully characterise this population.

Westerlund 1 under the light of Gaia EDR3: Distance, isolation, extent, and a hidden population

“…Their average is −48.7 km s −1 , in excellent agreement with the Gaia DR2 value. Both values are also in good agreement with the RVs measured by Ritchie et al (2022) from a sample of putatively non-binary early-type stars (which average −43 km s −1 ), although we cannot make a direct comparison to ensure that their velocities are in the same system as those from Gaia. Given the correction to the Local Standard of Rest in this direction (+4.1 km s −1 ), we can conclude that the velocity of the cluster is not very far away from v LSR = −43 km s −1 .…”

EWOCS-I: The catalog of X-ray sources in Westerlund 1 from the Extended Westerlund 1 and 2 Open Clusters Survey

“…Westerlund 1 is rich in binary systems. A high binary fraction has been identified in massive stars through spectroscopic (Ritchie et al 2022), radio (Dougherty et al 2010), infrared (Crowther et al 2006), and X-ray (Skinner et al 2006;Clark et al 2008Clark et al , 2019 observations. For instance, the WR population of Westerlund 1 has an estimated binary fraction of at least 70% (Crowther et al 2006;Clark et al 2008).…”

The Arches cluster revisited – IV. Observational constraints on the binary properties of very massive stars

“…Given our use of observed spectra of each target as templates, our RVs are necessarily relative rather than absolute. However, our approach to detecting variability in a target relies upon the maximum significance of an RV difference between any two pairs of epochs, as used by various authors (Sana et al 2013a ;Dunstall et al 2015 ;Patrick et al 2019Patrick et al , 2020Ritchie et al 2022 ):…”

“…The latter threshold was chosen to minimize contamination from pulsating sources within the sample and was moti v ated by the behaviour of blue super-/hypergiants within Westerlund 1 (Ritchie et al 2009 ). A higher 25 km s −1 threshold is adopted by Ritchie et al ( 2022 ) to accommodate the larger pulsational amplitudes observed in the early-B supergiant population of Westerlund 1, but these later-type objects are not present in the Arches. Foreshadowing Section 3 , we find no evidence for secular variability in the spectral morphologies of our targets, as is observed in the lower-mass blue super-/hypergiant contingent within Westerlund 1 (Ritchie et al 2009 ; ), and we regard the 20 km s −1 threshold adopted by Sana et al ( 2013a ) as appropriate for this study.…”

“…Explicitly excluding the post-supernova X-ray binary HD153919, of those systems for which (lower limits to) dynamical mass estimates are available, it is apparent that the secondaries are also highly luminous stars; such a preponderance of massive companions is also seen in both 30 Dor (Almeida et al 2017 ) and Westerlund 1 (Clark et al 2020 ;Ritchie et al 2022 ). This is likely to be in part an observational bias; in order for mass determinations to be made one requires SB2 systems.…”