Constraints on massive star formation: Cygnus OB2 was always an association

Nicholas J. Wright, Richard J. Parker, Simon P. Goodwin, Jeremy J. Drake

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We examine substructure and mass segregation in the massive OB association Cygnus OB2 to better understand its initial conditions. Using a well-understood Chandra X-ray selected sample of young stars, we find that Cyg OB2 exhibits considerable physical substructure and has no evidence for mass segregation, both indications that the association is not dynamically evolved. Combined with previous kinematical studies we conclude that Cyg OB2 is dynamically very young, and what we observe now is very close to its initial conditions: Cyg OB2 formed as a highly substructured, unbound association with a low volume density (<100 stars pc-3). This is inconsistent with the idea that all stars form in dense, compact clusters. The massive stars in Cyg OB2 show no evidence for having formed particularly close to one another, nor in regions of higher than average density. Since Cyg OB2 contains stars as massive as ∼100 M, this result suggests that very massive stars can be born in relatively low-density environments. This would imply that massive stars in Cyg OB2 did not form by competitive accretion, or by mergers.

Original languageEnglish
Article numberstt2232
Pages (from-to)639-646
Number of pages8
JournalMonthly Notices of the Royal Astronomical Society
Issue number1
Publication statusPublished - 1 Feb 2014


  • Open clusters and associations: individual: Cygnus OB2
  • Stars: formation
  • Stars: kinematics and dynamics


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