Dynamically triggered star formation in giant molecular clouds

A Lagrangian, particle-based numerical method (tree code gravity plus smoothed particle hydrodynamics) was used to simulate clump-clump collisions occurring within GMCs. The collisions examined were between 75M clumps at a relative Mach number ofM = 3. The clumps were modelled using isothermal spheres which were individually in stable equilibrium. The collisions formed shock-compressed layers, out of which condensed approximately co-planar protostellar discs of 7-60M mass and 500-1000AU radius. Binary and multiple systems were the usual final state. Lower mass objects were also produced, but commonly underwent disruption or merger. Such objects occasionally survived by being ejected via a three-body slingshot event resulting from an encounter with a binary system. The impact parameter b denotes how offset the clumps are from one another, with low values corresponding to near-head on collisions, and high values corresponding to grazing collisions. Varying b altered the processes by which the protostellar systems formed. At low b a single central disc formed initially, and was then spun-up by an accretion flow, causing it to produce secondaries via rotational instabilities. At mid b the shocked layer which formed initially broke up into fragments, and discs were then formed via fragment merger. At large b single objects formed within the compressed leading edge of each clump. These became unbound from each other as b was increased further. The effect of changing numerical factors was examined by : (i) colliding clumps which had been re-oriented before the collision (thus altering the initial particle noise), and (ii) by quadrupling the number of particles in each clump (thus increasing the resolution of the simulation). Both changes were found to affect the small-scale details of a collision, but leave the large scale morphology largely unaltered. It was concluded that clump-clump collisions provide a natural mechanism by which multiple protostellar systems may form.