Three-Dimensional Numerical Simulation of Interstellar Cloud-Cloud Collisions and Triggered Star Formation. I

We simulate the head· on collisions between isothermal interstellar clouds by using a three· dimensional hydrodynamic code. The cloud·cloud collision is one of the important mechanisms which trigger star formation. We obtain a new dynamical criterion for the gravitational instability instead of the static criterion for the Bonnor-Ebert sphere, In the collision between identical clouds with relative velocity greater than the sound velocity, the shock compressed layer is formed and the central density increases by the square of the impact Mach number. As a result, the enhanced self-gravity induces the instability and the central part of the cloud begins to collapse even if the total ,