EPoS Contribution
EPoS Contribution
Not all stars form in clusters: Gaia reveals how the dynamics of OB associations are inherited from the ISM

Diederik Kruijssen
U Heidelberg, Heidelberg, DE
Historically, it has often been asserted that most massive stars form in compact clusters. In this 'monolithic' scenario, present-day gravitationally-unbound OB associations are the result of the expansion of initially gravitationally-bound clusters of massive stars. The alternative is a hierarchical picture of star formation, in which stars are formed across a continuous distribution of gas densities and most OB associations never were bound clusters. In this scenario, they are formed in-situ, as the low-density side of this distribution, rather than as the remnants of expanding clusters. We utilise the second Gaia data release to quantify the degree to which OB associations are undergoing expansion and, therefore, whether OB associations are the product of expanding clusters, or whether they were born in-situ, as the large-scale, globally-unbound associations that we see today. We find that the observed kinematic properties of associations are consistent with highly substructured velocity fields and additionally require some degree of localised expansion from sub-clusters within the association. While most present-day OB associations do exhibit low levels of expansion, there is no significant correlation between radial velocity and radius. Therefore, the large-scale structure of associations is not set by the expansion of clusters, rather it is inherited from the molecular gas clouds from which the associations were formed. This finding is inconsistent with a monolithic model of association formation and instead favours a hierarchical model, in which OB associations form in-situ, following the fractal structure of the gas from which they form. This work demonstrates the huge discovery potential of current and future Gaia releases for understanding how ISM dynamics map to the kinematics of young stellar populations.
Caption: Velocity vector map based on one of the observed OB associations. The observed velocity field is modified by (1) randomising the directions of the velocity vectors, (2) making them locally correlated to generate co-moving groups (which manifests itself most clearly for the overdensity near the centre), and (3) adding expanding substructures (shown in red). The kinematic properties of the resulting OB association are statistically indistinguishable from the observations. This shows that the dynamics of OB associations reflect correlated random motion (as expected for motion inherited from the hierarchically structured ISM) with a couple of small-scale expansion events. OB associations that arise by the large-scale expansion from one or more compact clusters are qualitatively ruled out by the Gaia data.
Collaborators:
J. Ward, Heidelberg, DE
H.-W. Rix, MPIA, DE
Key publication

Suggested Session: High-Mass Star Formation