The structure and dynamics of the Vela OB2 association

Abstract

This thesis presents studies of the structure and dynamics of the Vela OB2 association. OB associations are sparse, gravitationally unbound groups of young stars, vital for studying the formation and early evolution of stars and star-forming regions, and the formation of the Galactic field population. The majority of stars in our Galaxy and others form in associations or clusters, ranging in size from hundreds of stars to tens of thousands, but only a small fraction of these groups are dense enough to remain together for a long time (Lada & Lada 2003). OB associations represent the transitional phase of these groups between formation from turbulent molecular clouds to dispersion into the Galactic field. And it is in these early stages of a star’s life that their subsequent evolution is determined and in which they form planetary systems.
We used Gaia DR1 and 2MASS photometry to trace the wide-scale population of YSOs (young stellar objects) across Vela OB2, using the GES (Gaia ESO Survey) sample of the Vel cluster as a basis. We plot a YSO density map and select targets for two observing runs, complementing it with Gaia DR2 photometry and astrometry for the second observing run. The spectra were used to obtain radial velocities (RVs) and Lithium equivalent-widths (EW(Li)s), the latter used to confirm the youth of our targets. We used the first set of spectra to study the A and B populations identified by Jeffries et al. (2014), expanding on their sample. We investigated the kinematics of the two groups and found that while the Vel cluster (population A) did not show expansion, the wider Vela OB2 sources (population B) does show significant evidence for expansion.
The second set of spectra provided a sample of ~400 YSOs across Vela OB2 with 3D spatial and kinematic data. We identify significant substructure and separate the sample into six groups, including the Vela OB2 association and three open clusters. We find significant evidence of expansion in the association with a kinematic age of 13-23 Myr, in good agreement with literature ages of 15-20 Myr. We also trace back the motion of the association and the clusters, constraining their arrangement at formation. Finally, we identified a number of candidate runaways stars, and show that by using RV and age estimates for the stars we can rule out the majority of these as runaways from known clusters.