Dynamically triggered star formation in giant molecular clouds

A Lagrangian, particle-based numerical method (tree-code gravity plus smoothed particle hydrodynamics) is used to simulate clump-clump collisions occurring within giant molecular clouds. The collisions examined are between 75-M. clumps at a relative Mach number of M = 3. The clumps are modelled using isothermal spheres which are individually in stable equilibrium. The collisions form shock-compressed layers, out of which condense approximately coplanar protostellar discs of 7-60M. mass and 500-1000 au radius. Binary and multiple systems are the usual final state. Lower mass objects are also produced, but commonly undergo disruption or merger. Such objects occasionally survive 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 alters the processes by which the protostellar systems form. At low b a single central disc forms initially, and is then spun up by an accretion flow, causing it to produce secondaries via rotational instabilities. At mid b the shocked layer which forms initially breaks up into fragments, and discs are then formed via fragment mergers. At large b single objects form within the compressed leading edge of each clump. These become unbound from each other as b is increased further. The effect of changing numerical factors is examined by (i) colliding clumps that have been re-oriented before the collision (thus altering the initial particle noise), and (ii) quadrupling the number of particles in each clump (thus increasing the resolution of the simulation). Both changes are found to affect the small-scale details of a collision, but leave the large-scale morphology largely unaltered. It is concluded that clump-clump collisions provide a natural mechanism by which multiple protostellar systems may form.