Gravitational Instability Induced by a Cloud-Cloud Collision: The Case of Head-on Collisions between Clouds with Different Sizes and Densities

Abstract

In order to investigate star formation triggered by cloud-cloud collisions, we performed axisymmetric numerical hydrodynamic calculations of supersonic, head-on collisions between non-identical clouds. Using axisymmetric smoothed particle hydrodynamics, we studied the condition of the gravitational instability of clouds. Non-identical cloud-cloud collisions, that is, collisions between clouds with different sizes and densities, are more frequent in interstellar space. We assumed that a small cloud is denser than a large cloud, and that both small and large clouds are gravitationally stable before collision. We examined two types of initial cloud models: hydrostatic equilibrium clouds confined by the external pressure and uniform density clouds. Our numerical results show that the large cloud is disrupted due to a bow shock induced by the collision, and that the small cloud is compressed by the shock. The self-gravity is enhanced in the postshock gas of the small cloud and a compact clump is tentatively formed. We describe the case in which this clump becomes gravitationally unstable. We have found that the initial mass of a small cloud needed to be gravitational unstable by the non-identical collisions is well below the Jeans mass of the initial small clouds. In the hydrostatic cloud model, a non-identical cloud-cloud collision can more easily induce a gravitational instability than in the uniform cloud model. We discuss the important role of non-identical cloud-cloud collisions in burst star formation in galaxies.