Revisiting the Cygnus OB associations

Alexis L Quintana,Nicholas J Wright

doi:10.1093/mnras/stab2663

Abstract

ABSTRACT OB associations play an important role in Galactic evolution, though their origins and dynamics remain poorly studied, with only a small number of systems analysed in detail. In this paper, we revisit the existence and membership of the Cygnus OB associations. We find that of the historical OB associations only Cyg OB2 and OB3 stand out as real groups. We search for new OB stars using a combination of photometry, astrometry, evolutionary models, and an SED-fitting process, identifying 4680 probable OB stars with a reliability of &gt;90 per cent. From this sample, we search for OB associations using a new and flexible clustering technique, identifying six new OB associations. Two of these are similar to the associations Cyg OB2 and OB3, though the others bear no relationship to any existing systems. We characterize the properties of the new associations, including their velocity dispersions and total stellar masses, all of which are consistent with typical values for OB associations. We search for evidence of expansion and find that all are expanding, albeit anisotropically, with stronger and more significant expansion in the direction of Galactic longitude. We also identify two large-scale (160 pc and 25 km s−1) kinematic expansion patterns across the Cygnus region, each including three of our new associations, and attribute this to the effects of feedback from a previous generation of stars. This work highlights the need to revisit the existence and membership of the historical OB associations, if they are to be used to study their properties and dynamics.

Highlights

Many OB stars are assembled in associations (McKee & Williams 1997)
How do our new groups compare to the historical associations discussed in Section 2? We compared the membership of our groups with that of the historical associations, our results confirming what we found in Section 2: while Cyg OB2 stands out as a real group and Cyg OB3 has some overlap with group A, the same is not true for the other groups, which show very little overlap with the historical OB associations
We have investigated the existence of the OB associations in Cygnus, concluded that the current associations, on the whole, do not represent kinematically coherent groups of stars, and identified six new groups of OB stars that appear more coherent and are likely to be real OB associations

Summary

Introduction

They were first defined by Ambartsumian (1947) as gravitationnally unbound groups of young stars with a high concentration of OB stars They typically have total masses ranging from a few thousands to a few tens of thousands of stellar masses, with a density lower than 0.1 M pc−3 and a young age (necessary to explain their kinematic and spatial concentration despite their unbound nature). They often extend over tens of parsecs and include smaller groups of different ages and kinematics (Garmany 1994; Wright 2020). As a transitional phase between star-forming regions and the field population of stars, OB associations are important to study in order to better understand Galactic evolution (Wright 2020)

Objectives

Discussion

Conclusion