Supersonic cloud collision. I

Abstract

It has long been suggested that shocks might play an important role in altering the form of the interstellar medium (ISM). Shocks enhance gas density and sufficiently dense regions may become self gravitating. Potential star-forming clouds within larger molecular clouds, move randomly at supersonic speeds and a collision between them, is highly inelastic.

Depending on the precollision velocity, colliding molecular clouds produce a slab that is either shock-compressed or pressure-confined. In a sequel of two Papers (I and II), we simulate molecular cloud collision and investigate the dynamical evolution of such slabs. Shocked slabs appear susceptible to hydrodynamic instabilities and, in the present Paper (I), we study the effect of strong shear between slab layers on the dynamic evolution of a shock compressed gas slab. Head-on and off-centre cloud collisions have been examined in this work. Self gravity is included in all our simulations.

Simulations presented were performed using the smoothed particle hydrodynamics (SPH) numerical scheme. Individual, precollision clouds are modelled as pressure-confined Bonnor-Ebert spheres. However, for simplicity the thermodynamic details of the problem are simplified and the gas temperature is evolved simply by a barytropic equation of state. Obviously, the gas suffers to some extent from thermal inertial effects. However, we note that the dynamical timescale is much smaller than the local sound crossing time so that any such effects should have minimum influence.

Strong shocks are highly radiative. Thus a highly supersonic cloud collision produces a cold, roughly isothermal shock compressed gas slab. We find that the shocked slab is susceptible to dynamical instabilities like the gravitational instability, Kelvin-Helmholtz (KH) instability and the non linear thin shell instability (NTSI). Rapid growth of instabilities within the slab produces structure in it. The NTSI competes with the gravitational instability and the fate of the shocked slab apparently depends on the relative dominance of either of the two instabilities. Dominance of the NTSI causes turbulent mixing between slab layers and dissipates internal energy. Eventually the slab collapses to form a thin elongated body, aligned with the collision axis, and star formation may commence in it. Our hydrodynamical models discussed here suggest that, high-velocity cloud collisions may be a viable mechanism for the formation of observed filamentary structure in the ISM.