Energy Dissipation in Interstellar Cloud Collisions

We present a study of the kinetic energy dissipation in interstellar cloudcollisions. The main aim is to understand the dependence of the elasticity(defined as the ratio of the final to the initial kinetic energy of the clouds)on the velocity and mass ratio of the colliding clouds, magnetic fieldstrength, and gas metallicity for head-on collisions. The problem has beenstudied both analytically and via numerical simulations. We have derived handyanalytical relationships that well approximate the analogous numerical results.The main findings of this work are: (i) the kinetic energy dissipation in cloudcollisions is minimum (i.e. the collision elasticity is maximum) for a cloudrelative velocity $v_r \simeq 30 km s^{-1}$; (ii) the above minimum value isproportional $Z L_c^2$, where $Z$ is the metallicity and $L_c$ is the cloudsize: the larger is $Z L_c^2$ the more dissipative (i.e. inelastic) thecollision will be; (iii) in general, we find that the energy dissipationdecreases when the magnetic field strength, and mass ratio of the clouds areincreased and the metallicity is decreased, respectively. We briefly discussthe relevance of this study to the global structure of the interstellar mediumand to galaxy formation and evolution.Comment: 16 pages, aasms LaTeX, 7 figures. ApJ, accepte