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 colli sions occurring within giant molecular clouds. The collisions examined are between 75-M. clumps at a relative Mach number of M = 3. The clum ps are modelled using isothermal spheres which are individually in sta ble equilibrium. The collisions form shock-compressed layers, out of w hich condense approximately coplanar protostellar discs of 7-60M. mass and 500-1000 au radius. Binary and multiple systems are the usual fin al state. Lower mass objects are also produced, but commonly undergo d isruption or merger. Such objects occasionally survive by being ejecte d via a three-body slingshot event resulting from an encounter with a binary system. The impact parameter b denotes how offset the clumps ar e from one another, with low values corresponding to near-head-on coll isions, 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 accr etion flow, causing it to produce secondaries via rotational instabili ties. 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 ef fect of changing numerical factors is examined by (i) colliding clumps that have been re-oriented before the collision (thus altering the in itial particle noise), and (ii) quadrupling the number of particles in each clump (thus increasing the resolution of the simulation). Both c hanges are found to affect the small-scale details of a collision, but leave the large-scale morphology largely unaltered. It is concluded t hat clump-clump collisions provide a natural mechanism by which multip le protostellar systems may form.