High Velocity Oblique Cloud Collision and Star and Star Cluster Formation through Gravitational Instability of the Shock-Compressed Slab with Rotation and Velocity Shear

Abstract

We study the gravitational instability of an isothermal gaseous slab formed by cloud-cloud collision and compression at the cloud interface. The compressed gaseous slab rotates and has velocity shear except when the collision is not exactly head-on. The effects of the rotation and velocity shear on the gravitational instability have been evaluated for the first time. We obtained the growth rate of perturbations as a function of the wavelength for various gaseous slab models. We also obtained the changes in the density and velocity due to the perturbations. Two types of unstable modes are found; one is due to the self-gravity of the gaseous slab and the other is due to the velocity shear. The former instability leads to the formation of star and star clusters. Velocity shear decreases the growth rate of the former instability while it increases that of the latter instability. Rotation also decreases the growth of the former instability. As the slab grows in mass, the growth rate of the self-gravitational instability increases and eventually becomes greater than the growing timescale of the slab. Then a gravitationally bound condensation is formed and its mass is larger when the slab has stronger velocity shear and rotation.