SPH simulations of clumps formation by dissipative collision of molecular clouds - I. Non magnetic case

Computer experiments of interstellar cloud collisions were performed with a new smoothed-particle-hydrodynamics (SPH) code. The SPH quantities were ca lculated by using spatially adaptive smoothing lengths and the SPH fluid eq uations of motion were solved by means of a hierarchical multiple time-scal e leapfrog. Such a combination of methods allows the code to deal with a la rge range of hydrodynamic quantities. A careful treatment of gas cooling by H, H-2, CO and H II, as well as a heating mechanism by cosmic rays and by H-2 production on grains surface, were also included in the code. The gas m odel reproduces approximately the typical environment of dark molecular clo uds. The experiments were performed by impinging two dynamically identical spherical clouds onto each other with a relative velocity of 10 km s(-1) bu t with a different impact parameter for each case. Each object has an initi al density profile obeying an r(-1)-law with a cutoff radius of 10 pc and w ith an initial temperature of 20 K. As a main result, cloud-cloud collision triggers fragmentation but in expense of a large amount of energy dissipat ed, which occurred in the head-on case only. Off-center collision did not a llow remnants to fragment along the considered time (similar to 6 Myr). How ever, it dissipated a considerable amount of orbital energy. Structures as small as 0.1 pc, with densities of similar to 10(4) cm(-3), were observed i n the more energetic collision.