If it does not kill them, it makes them stronger: collisional evolution of star clusters with tidal shocks
(8 votes from 7 institutions)
The radii of young (< 100 Myr) star clusters correlate only weakly with their masses. This shallow relation has been used to argue that impulsive tidal perturbations, or `shocks', by passing giant molecular clouds (GMCs) preferentially disrupt low-mass clusters. We show that this mass-radius relation is in fact the result of the combined effect of two-body relaxation and repeated tidal shocks. Clusters evolve towards a typical radius of a few parsecs, as observed, independent of the initial radius and this radius is set by a competition between expansion by relaxation and shrinking due to shocks. The equilibrium cluster density is higher for stronger tidal shocks, such that disruption rates in gas-rich galaxies in the early Universe are only a factor of a few higher than in the solar neighbourhood. We conclude that interactions with GMCs are not a viable explanation for the near universality of the globular cluster mass function, as was suggested by Elmegreen and Kruijssen.