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Massive stars in the hinterland of the young cluster, Westerlund 2

Drew, J. E. and Herrero, A. and Mohr-Smith, M. and Monguió, M. and Wright, N. J. and Kupfer, T. and Napiwotzki, R. (2018) Massive stars in the hinterland of the young cluster, Westerlund 2. Monthly Notices of the Royal Astronomical Society, 480 (2). pp. 2109-2124. ISSN 0035-8711.

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An unsettled question concerning the formation and distribution of massive stars is whether they must be born in massive clusters and, if found in less dense environments, whether they must have migrated there. With the advent of wide-area digital photometric surveys, it is now possible to identify massive stars away from prominent Galactic clusters without bias. In this study we consider 40 candidate OB stars found in the field around the young massive cluster, Westerlund 2, by Mohr-Smith et al.: these are located inside a box of 1.5 × 1.5 deg^2 and are selected on the basis of their extinctions and K magnitudes. We present VLT/X-shooter spectra of two of the hottest O stars, respectively 11 and 22 arcmin from the centre of Westerlund 2. They are confirmed as O4V stars, with stellar masses likely to be in excess of 40 M_⊙. Their radial velocities relative to the non-binary reference object, MSP 182, in Westerlund 2 are −29.4 ± 1.7 and −14.4 ± 2.2 km s^(−1), respectively. Using Gaia DR2 proper motions we find that between 8 and 11 early O/WR stars in the studied region (including the two VLT targets, plus WR 20c and WR 20aa) could have been ejected from Westerlund 2 in the last one million years. This represents an efficiency of massive-star ejection of up to ∼ 25 per cent. On sky, the positions of these stars and their proper motions show a near N–S alignment. We discuss the possibility that these results are a consequence of prior sub-cluster merging combining with dynamical ejection.

Item Type:Article
Related URLs:
URLURL TypeDescription Paper
Drew, J. E.0000-0003-1192-7082
Kupfer, T.0000-0002-6540-1484
Additional Information:© 2018 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society. This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model ( Accepted 2018 July 13. Received 2018 July 13; in original form 2018 June 1. This paper is based on data products from observations made with ESO Telescopes at the La Silla Paranal Observatory under programme ID 095.D-0843. It also rests on results obtained directly from the VST Photometric Hα Survey of the Southern Galactic Plane and Bulge (VPHAS+,; ESO programme 177.D-3023). In addition, use is made of data products from the Two Micron All Sky Survey (2MASS), which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, and of data products from the Wide-field Infrared Survey Explorer (WISE), which is a joint project of the University of California, Los Angeles, and the Jet Propulsion Laboratory/California Institute of Technology. Both 2MASS and WISE have benefitted from funding by the National Aeronautical and Space Administration. Finally, we have used data from the European Space Agency mission Gaia (, processed by the Gaia Data Processing and Analysis Consortium (DPAC, Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement. The analysis in this paper has made use of TopCat (Taylor 2005). JED and MM acknowledge the support of a research grant funded by the Science, Technology and Facilities Council of the UK (STFC, ref. ST/M001008/1). AH acknowledges support from Spanish MINECO under project grants AYA2015-68012-C2-1 and SEV2015-0548 and from the Gobierno de Canarias under project grant ProID2017010115. NJW acknowledges receipt of an STFC Ernest Rutherford Fellowship (ref. ST/M005569/1). We thank an anonymous referee for comments that have helped improve this paper’s content.
Funding AgencyGrant Number
Gaia Multilateral AgreementUNSPECIFIED
Science and Technology Facilities Council (STFC)ST/M001008/1
Ministerio de Economía y Competitividad (MINECO)AYA2015-68012-C2-1
Ministerio de Economía y Competitividad (MINECO)SEV2015-0548
Gobierno de CanariasProID2017010115
Science and Technology Facilities Council (STFC)ST/M005569/1
Subject Keywords:surveys, stars: early-type, open clusters and associations: individual: Westerlund 2, Galaxy: structure
Issue or Number:2
Record Number:CaltechAUTHORS:20181205-163501449
Persistent URL:
Official Citation:J E Drew, A Herrero, M Mohr-Smith, M Monguió, N J Wright, T Kupfer, R Napiwotzki; Massive stars in the hinterland of the young cluster, Westerlund 2, Monthly Notices of the Royal Astronomical Society, Volume 480, Issue 2, 21 October 2018, Pages 2109–2124,
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:91525
Deposited By: George Porter
Deposited On:07 Dec 2018 15:37
Last Modified:03 Oct 2019 20:35

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