Self-Gravitational Instability of an Isothermal Gaseous Slab under High External Pressure

Abstract

We carried out two-dimensional hydrodynamic simulations to study the nonlinear growth of the gravitational instability of a self-gravitating isothermal gaseous slab confined by external pressure. When the external pressure is low, the Jeans instability creates dense collapsing clumps for long-wavelength perturbations. When the external pressure is high (Pext/P00 > 0.64 where P00 is the initial mid-plane pressure and Pext is the external pressure), an incompressible mode of gravitational instability creates clumps, the mass of which is smaller than the Jeans mass. The clumps merge due to mutual gravitational attraction. Although each clump cannot collapse gravitationally, when its mass exceeds the Jeans mass by merging, the merged clump collapses gravitationally. These results indicate that when the external pressure is high, star formation can be triggered by the merging of low-mass Jeans stable clumps. In high external pressure regions, we expect that the mass function of collapsing clumps in the clouds has a peak around the Jeans mass.