Collision between dissimilar clouds: stability of the bow-shock, and the formation of pre-stellar cores

Abstract

We use smoothed particle hydrodynamics (SPH) to simulate a head-on collision between two physically dissimilar clouds, and investigate the dynamical stability of the post-collision bow-shock. The shock-front appears susceptible to a number of hydrodynamical instabilities such as the Kelvin–Helmholtz instability, the Rayleigh–Taylor instability, and the thin shell instability (TSI). We initially perform three realizations of the simulation by progressively increasing the number of SPH particles, and therefore the resolution. It is observed that lack of sufficient resolution tends to damp the growth of perturbations on the shock-front which in turn suppresses fragmentation. Thus poorer resolution favours formation of contiguous structure while the fingers, characteristic of the Rayleigh–Taylor instability, hardly appear. However, the gross physical features seem independent of resolution, the bow-shock in each of the three cases becomes unstable to the TSI which once triggered, grows rapidly and eventually becomes non-linear. Albeit, the TSI appears to grow at a rate much slower than that predicted analytically. The TSI contributes to dissipation of gas kinetic energy within the curved shock-front via internal shocking, eventually the bow-shock collapses to form a filament oriented along the collision axis; pre-stellar cores form in this filament. The wings of the bow-shock are also unstable to the TSI and consequently appear filamentary with a few clumps, which rapidly disrupt over a period of a few times 105 yr. Formation of such transitional clumps may explain the occurrence of starless cores often found in filamentary regions. Having commenced with perfectly stable initial conditions, the reported instabilities in the bow-shock originate purely through numerical noise. Turbulence within the shock-front, however, appears to locally suppress the gravitational instability. We demonstrate that a global gravitational contraction can produce turbulence, large enough to contain pre-stellar cores. Finally, we also discuss a case in which the initial density contrast between the pre-collision clouds was further increased by an order of magnitude, while colliding them at a much lower velocity.