Cygnus OB2 DANCe: A high-precision proper motion study of the Cygnus OB2 association

Abstract

We present a high-precision proper motion study of 873 X-ray and spectroscopically selected stars in the massive OB association Cygnus OB2 as part of the DANCe project. These were calculated from images spanning a 15 year baseline and have typical precisions < 1 mas/yr. We calculate the velocity dispersion in the two axes to be σ α (c) = 13.0 +0.8 −0.7 and σ δ (c) = 9.1 +0.5 −0.5 km s −1 , using a 2-component, 2-dimensional model that takes into account the uncertainties on the measurements. This gives a 3-dimensional velocity dispersion of σ 3D = 17.8 ± 0.6 km s −1 implying a virial mass significantly larger than the observed stellar mass, confirming that the association is gravitationally unbound. The association appears to be dynamically unevolved, as evidenced by considerable kinematic substructure, non-isotropic velocity dispersions and a lack of energy equipartition. The proper motions show no evidence for a global expansion pattern, with approximately the same amount of kinetic energy in expansion as there is in contraction, which argues against the association being an expanded star cluster disrupted by process such as residual gas expulsion or tidal heating. The kinematic substructures, which appear to be close to virial equilibrium and have typical masses of 40-400 M ⊙ , also do not appear to have been affected by the expulsion of the residual gas. We conclude that Cyg OB2 was most likely born highly substructured and globally unbound, with the individual subgroups born in (or close to) virial equilibrium, and that the OB association has not experienced significant dynamical evolution since then.

The kinematics of the Scorpius-Centaurus OB association from Gaia DR1

“…Taken together these results suggest that while there is kinematic substructure in each of the three subgroups of Sco-Cen, the level of kinematic substructure varies between subgroups and is lower than that measured in Cyg OB2 (Wright et al 2016).…”

“…We use this instead of Geary's C index (Geary 1954), since Moran's I statistic is a global measure that can identify and quantify large-scale spatial correlation (as appears to be evident in Figure 11), while Geary's C is a local measure of spatial correlation. For brevity we don't provide quantification of the index, but instead refer the reader to these papers and Wright et al (2016).…”

“…It may manifest as sizeable substructures within the association (e.g., the Gamma Velorum cluster shows two distinct kinematic components, Jeffries et al 2014) or as smaller, more numerous substructures (as observed in Cygnus OB2, Wright et al 2016). In the case of OB associations, kinematic substructure is believed to be a remnant of the primordial phase-space structure that was present when the association formed (e.g., Larson 1981;Wright et al 2016). In this Section we explore the evidence for kinematic substructure in Sco-Cen.…”

“…In this Section we explore the evidence for kinematic substructure in Sco-Cen. Figure 12 shows a corrected proper motion vector map for Sco-Cen with the vectors colour-coded by the position angle of their motion on the celestial sphere (as per Wright et al 2016). In this figure the three subgroups stand out relatively clearly due to their different kinematics, with some overlap between the groups that suggests their membership could benefit from some adjustments.…”

“…Wright et al (2016) performed the first such study, using ground-based proper motions to study the kinematics of the Cygnus OB2 association. They found that the association lacked the coherent radial expansion pattern expected if it was an expanding star cluster.…”