School of Physical and Chemical Sciences, Te Kura Matū Seminar Series

Not all stars form in clusters - measuring the kinematics of OB associations with Gaia


Dr Jacob (Jake) Ward


Center for Astronomy, Department of Physics & Astronomy, University of Heidelberg

Time & Place

Fri, 20 Apr 2018 11:00:00 NZST in Room 531, Level 5, West Building

All are welcome


It is often stated 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 massive, gravitationally bound star 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 is consistent with a monolithic star formation scenario. We compare the distributions of each of these diagnostics with those derived from model OB associations with kinematics representative of random motions and expansion, both from a single cluster and from multiple clusters. None of these diagnostics show any evidence of expansion, and the observed kinematics are best represented by a random velocity distribution. This result favours the hierarchical star formation model in which a minority of stars forms in bound clusters and large-scale, hierarchically-structured associations are formed in-situ.

I will discuss the implications of these results with particular emphasis on unique potential of Gaia to distinguish between the monolithic and hierarchical star formation paradigms, as well as discussing the limitations of the existing data. Finally, I will discuss the potential for combining future Gaia data releases with data from radial velocity surveys to perform a 6-dimensional analysis of the nature and origin of OB associations in our Galaxy.



Further information: Dr Jacob WardUniversity of Heidelberg