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

TL;DR

This study investigates the presence and origins of massive stars around Westerlund 2, confirming some as ejected from the cluster and exploring their distribution and dynamics.

Contribution

It provides spectroscopic confirmation of massive stars outside Westerlund 2 and analyzes their potential ejection origins using Gaia data.

Findings

Confirmed two O4V stars as cluster members 11 and 22 arcmin away.

Estimated up to 25% of massive stars in the region may have been ejected.

Stars show a near N--S proper motion alignment, suggesting dynamical ejection or sub-cluster merging.

Abstract

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 (2017): these are located inside a box of 1.5x1.5 square degrees 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 Msun. 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, 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%. 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.