3-D Numerical Hydrodynamics of Self-Gravitating Gas : Collisions and Fragmentations

Abstract

The collisions between self-gravitating gas (e.g. interstellar clouds or stars) are investigated using the three dimensional Smoothed Particle Hydrodynamics. We solve the Euler equation coupled with the Poisson equation numerically. The formalism to calculate the energy equation and to include the radiation reaction of gravitational waves are also presented. In the gas system with Newtonian gravity, the criterion for gravitational instability is known as the Jeans criterion by linear perturbation theory. We simulate the nonlinear evolution and find the dynamical criterion different from that. In supersonic head-on collisions between two stable isothermal clouds, the shock compression increases the density and the self-gravity can trigger the instability or induce the quadrupole oscillation as expected in the tensor virial analysis. When we include the gas cooling effect, the cloud fragments into small pieces. In the case of off-center collisions, the outcomes depend on the nondimensional constant q ≡ JC_s/GM^2, where M is the total mass, J is the total angular momentum and C_s is the sound velocity of isothermal gas. If the parameter is small, q ≲ 0.2, the shock compression triggers the gravitational collapse and the rapidly rotating core forms near the collisional center. The system with q ≳ 0.4 starts fission to form the binary cloud system after the collisional merging, For the intermediate case , they make a merged disk with a bar-spiral structure.