Star formation triggered by cloud–cloud collisions

We present the results of smoothed particle hydrodynamics simulations in which two clouds, each having mass MO = 500 M⊙ and radius RO = 2 pc, collide head-on at relative velocities of ΔvO = 2.4, 2.8, 3.2, 3.6 and 4.0 km s−1. There is a clear trend with increasing ΔvO. At low ΔvO, star formation starts later, and the shock-compressed layer breaks up into an array of predominantly radial filaments; stars condense out of these filaments and fall, together with residual gas, towards the centre of the layer, to form a single large-N cluster, which then evolves by competitive accretion, producing one or two very massive protostars and a diaspora of ejected (mainly low-mass) protostars; the pattern of filaments is reminiscent of the hub and spokes systems identified recently by observers. At high ΔvO, star formation occurs sooner and the shock-compressed layer breaks up into a network of filaments; the pattern of filaments here is more like a spider's web, with several small-N clusters forming independently of one another, in cores at the intersections of filaments, and since each core only spawns a small number of protostars, there are fewer ejections of protostars. As the relative velocity is increased, the mean protostellar mass increases, but the maximum protostellar mass and the width of the mass function both decrease. We use a Minimal Spanning Tree to analyse the spatial distributions of protostars formed at different relative velocities.