High-resolution simulations of clump-clump collisions using SPH with particle splitting

We investigate, by means of numerical simulations, the phenomenology of star formation triggered by low-velocity collisions between low-mass molecular clumps. The simulations are performed using a smoothed particle hydrodynamics code which satisfies the Jeans condition by invoking on-the-fly particle splitting. Clumps are modelled as stable truncated (non-singular) isothermal, i.e. Bonnor-Ebert, spheres. Collisions are characterized by M-0 (clump mass), b (offset parameter, i.e. ratio of impact parameter to clump radius) and M (Mach number, i.e. ratio of collision velocity to effective post-shock sound speed). The gas subscribes to a barotropic equation of state, which is intended to capture (i) the scaling of pre-collision internal velocity dispersion with clump mass, (ii) post-shock radiative cooling and (iii) adiabatic heating in optically thick protostellar fragments. The efficiency of star formation is found to vary between 10 and 30 per cent in the different collisions studied and it appears to increase with decreasing M-0, and/or decreasing b, and/or increasing M. For b < 0.5 collisions produce shock-compressed layers which fragment into filaments. Protostellar objects then condense out of the filaments and accrete from them. The resulting accretion rates are high, 1-5 X 10(-5)M(circle dot) yr(-1) , for the first 1-3 x 10(4) yr. The densities in the filaments, n(H2) greater than or similar to 5 x 10(5) cm(-3), are sufficient that they could be mapped in NH3 or CS line radiation, in nearby star formation regions.