Not all stars form in clusters: measuring the kinematics of OB associations with Gaia
Jacob Ward
Thursday December 5th, 18:00
It is often assumed that star clusters are the fundamental units of star formation and that most (if not all) stars form in dense stellar clusters. In this monolithic formation scenario, low density OB associations are formed from the expansion of gravitationally bound clusters following gas expulsion due to stellar feedback. From N-body simulations, one would expect associations that formed in this manner to continue to exhibit positive radial velocities and elevated levels of anisotropy over 10s of Myr. However, recent theoretical and observational studies suggest that star formation is a hierarchical process, following the fractal nature of natal molecular clouds and allowing the formation of large-scale associations in-situ. Using the Tycho-Gaia Astrometric Solution (TGAS) catalogue, we quantify four key kinematic diagnostics for 18 nearby OB associations in order to determine whether the typical kinematic behaviour of OB associations are consistent with a monolithic star formation scenario. Comparing the distributions of each of these diagnostics with those derived from model associations with kinematics representative of random motions and expanding velocity fields, we find no evidence to support the notion that nearby OB associations are undergoing gas-expulsion driven expansion, either from a single cluster or multiple clusters. With the second data release (Gaia DR2), Gaia now allows the kinematics of all nearby OB associations to be probed with unprecedented precision. I will present the exciting new results from the expansion of our study to Gaia DR2, with which we implement clustering algorithms to independently identify over-densities of OB-type stars before measuring the kinematic properties of over 100 potential OB associations. I will discuss the implications of these results of these studies with particular emphasis on the unique potential of Gaia to distinguish between the monolithic and hierarchical star formation paradigms.