Star formation triggered by non-head-on cloud–cloud collisions, and clouds with pre-collision sub-structure

Abstract

In an earlier paper, we used smoothed particle hydrodynamics (SPH) simulations to explore star formation triggered by head-on collisions between uniform-density 500M(circle dot) clouds, and showed that there is a critical collision velocity, v(CRIT). At collision velocities below vCRIT, a hub-and-spoke mode operates and delivers a monolithic cluster with a broad mass function, including massive stars (M-star greater than or similar to 10M(circle dot)) formed by competitive accretion. At collision velocities above vCRIT, a spider's-web mode operates and delivers a loose distribution of small sub-clusters with a relatively narrow mass function and no massive stars. Here we show that, if the head-on assumption is relaxed, vCRIT is reduced. However, if the uniform-density assumption is also relaxed, the collision velocity becomes somewhat less critical: a low collision velocity is still needed to produce a global hub-and-spoke system and a monolithic cluster, but, even at high velocities, large cores -capable of supporting competitive accretion and thereby producing massive stars - can be produced. We conclude that cloud-cloud collisions may be a viable mechanism for forming massive stars - and we show that this might even be the major channel for forming massive stars in the Galaxy.