The velocity structure of Cygnus OB2

TL;DR

This study investigates the velocity structure of Cygnus OB2, finding small-scale substructures but no evidence of overall expansion or contraction, and suggesting bound substructures exist despite the region being unbound.

Contribution

It provides new insights into the small-scale velocity substructure and the unbound nature of Cygnus OB2, contrasting with previous studies.

Findings

Stars within 0.5 parsecs have similar velocities

No evidence of expansion or contraction at any scale

Small-scale substructure exists despite the overall unbound state

Abstract

The kinematic structure of the Cygnus OB2 association is investigated. No evidence of expansion or contraction is found at any scale within the region. Stars that are within $\sim$ 0.5 parsecs of one another are found to have more similar velocities than would be expected by random chance, and so it is concluded that velocity substructure exists on these scales. At larger scales velocity substructure is not found. We suggest that bound substructures exist on scales of $\sim$ 0.5 parsecs, despite the region as a whole being unbound. We further suggest that any velocity substructure that existed on scales > 0.5 parsecs has been erased. The results of this study are then compared to those of other kinematic studies of Cygnus OB2.