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

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.

arXiv commentAccepted for publication in MNRAS, 13 July 2018. 16 pages, plus one-page table in an appendix

References in corpus (49)

• Binary interaction dominates the evolution of massive stars
• Local Kinematics and the Local Standard of Rest
• Grids of stellar models with rotation - I. Models from 0.8 to 120 Msun at solar metallicity (Z = 0.014)
• Trigonometric Parallaxes of High Mass Star Forming Regions: the Structure and Kinematics of the Milky Way
• Young massive star clusters
• A new calibration of stellar parameters of Galactic O stars
• Automated data reduction workflows for astronomy
• The High Angular Resolution Multiplicity of Massive Stars
• The Galactic O-Star Spectroscopic Survey (GOSSS). II. Bright Southern Stars
• Atmospheric NLTE-Models for the Spectroscopic Analysis of Blue Stars with Winds. II. Line-Blanketed Models
• On the origin of the O and B-type stars with high velocities II Runaway stars and pulsars ejected from the nearby young stellar groups
• A spectroscopic survey on the multiplicity of high-mass stars
• The empirical metallicity dependence of the mass-loss rate of O- and early B-type stars
• The IACOB project: I. Rotational velocities in Northern Galactic O and early B-type stars revisited. The impact of other sources of line-broadening
• The VST Photometric Halpha Survey of the Southern Galactic Plane and Bulge (VPHAS+)
• The Origin of Massive O-type Field Stars. Part II: Field O stars as runaways
• The Milky Way's rotation curve out to 100 kpc and its constraint on the Galactic mass distribution
• The Origin of OB Runaway Stars
• Molecular clouds towards RCW 49 and Westerlund 2; Evidence for cluster formation triggered by cloud-cloud collision
• WR 20a is an Eclipsing Binary: Accurate Determination of Parameters for an Extremely Massive Wolf-Rayet System
• The characteristics of high velocity O and B stars which are ejected from supernovae in binary systems
• Do O-stars form in isolation?
• Constraints on Massive Star Formation: Cygnus OB2 was always an Association
• Radial Distribution of Stellar Motions in Gaia DR2
• Temperature and Density Distribution in the Molecular Gas Toward Westerlund 2: Further Evidence for Physical Association
• The IACOB project. V. Spectroscopic parameters of the O-type stars in the modern grid of standards for spectral classification
• Field O stars: formed in situ or as runaways?
• Early-type stars in the core of the young open cluster Westerlund2
• The formation of young dense star clusters through mergers
• The Runaways and Isolated O-Type Star Spectroscopic Survey of the SMC (RIOTS4)
• Winds of low-metallicity OB-type stars: HST-COS spectroscopy in IC1613
• The VLT-FLAMES Tarantula Survey XXVI: Properties of the O-dwarf population in 30 Doradus
• The VLT-FLAMES Tarantula Survey. XIII: On the nature of O Vz stars in 30 Doradus
• The Distance to the Massive Galactic Cluster Westerlund 2 from a Spectroscopic and HST Photometric Study
• A High-Resolution Multiband Survey of Westerlund 2 With the Hubble Space Telescope I: Is the Massive Star Cluster Double?
• An X-ray Imaging Study of the Stellar Population in RCW49
• The mass of the very massive binary WR21a
• The deep OB star population in Carina from the VST Photometric H$\alpha$ Survey (VPHAS+)
• A spectroscopic investigation of early-type stars in the young open cluster Westerlund 2
• On the Distance and Molecular Environment of Westerlund 2 and HESS J1023-575
• Reddening, Distance, and Stellar Content of the Young Open Cluster Westerlund 2
• A High-Resolution Multiband Survey of Westerlund 2 With the Hubble Space Telescope. III. The present-day stellar mass function
• New OB star candidates in the Carina Arm around Westerlund 2 from VPHAS+
• Hot gas in the Wolf-Rayet nebula NGC3199
• Two O2 If*/WN6 stars possibly ejected from the massive young Galactic cluster Westerlund 2
• High resolution radio emission from RCW 49/Westerlund 2
• The IACOB spectroscopic database of Galactic OB stars
• A clustered origin for isolated massive stars
• A and F stars as probes of outer Galactic disc kinematics

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• Star clusters near and far; tracing star formation across cosmic time
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• The Gaia-ESO Survey: Asymmetric expansion of the Lagoon Nebula cluster NGC 6530 from GES and Gaia DR2
• SOFIA FEEDBACK survey: exploring the dynamics of the stellar wind driven shell of RCW 49
• The Villafranca catalog of Galactic OB groups: I. Systems with O2-O3.5 stars
• Unlocking Galactic Wolf-Rayet stars with $\text{Gaia}$ DR2 II: Cluster and Association membership
• The Massive Star-Forming Regions Omnibus X-Ray Catalog, Second Installment
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• The O star hinterland of the Galactic starburst, NGC 3603
• The Massive Star-Forming Regions Omnibus X-Ray Catalog, Third Installment
• Gaia and HST astrometry of the very massive $\sim$150 $M_{\odot }$ candidate runaway star VFTS682
• The early evolution of young massive clusters: The kinematic history of NGC6611 / M16
• Constraining the initial conditions of NGC 2264 using ejected stars found in Gaia DR2
• The young massive star cluster Westerlund 2 observed with MUSE. III. A cluster in motion -- the complex internal dynamics
• Proper motions of OB stars in the far Carina Arm
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• New Massive Contact Twin Binary in a Radio-quiet HII Region Associated with the M17 Complex
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• Radial velocities from far red spectra of Carina Arm O and early B stars
• Studying the ISM at $\sim$10 pc scale in NGC 7793 with MUSE -- II. Constraints on the oxygen abundance and ionising radiation escape
• Kinematics of the Central Stars Powering Bowshock Nebulae and the Large Multiplicity Fraction of Runaway OB Stars