Magnetohydrodynamics of Cloud Collisions in a Multiphase Interstellar Medium

We extend previous studies of the physics of interstellar cloud collisions bybeginning investigation of the role of magnetic fields through 2Dmagnetohydrodynamic (MHD) numerical simulations. We study head-on collisionsbetween equal mass, mildly supersonic diffuse clouds. We include a moderatemagnetic field and two limiting field geometries, with the field lines parallel(aligned) and perpendicular (transverse) to the colliding cloud motion. Weexplore both adiabatic and radiative cases, as well as symmetric and asymmetricones. We also compute collisions between clouds evolved through prior motion inthe intercloud medium and compare with unevolved cases. We find that: In the (i) aligned case, adiabatic collisions, like their HDcounterparts, are very disruptive, independent of the cloud symmetry. However,when radiative processes are taken into account, partial coalescence takesplace even in the asymmetric case, unlike the HD calculations. In the (ii)transverse case, collisions between initially adjacent unevolved clouds arealmost unaffected by magnetic fields. However, the interaction with themagnetized intercloud gas during the pre-collision evolution produces a regionof very high magnetic energy in front of the cloud. In collisions betweenevolved clouds with transverse field geometry, this region acts like a``bumper'', preventing direct contact between the clouds, and eventuallyreverses their motion. The ``elasticity'', defined as the ratio of the final tothe initial kinetic energy of each cloud, is about 0.5-0.6 in the cases weconsidered. This behavior is found both in adiabatic and radiative cases.