Magnetohydrodynamics of cloud collisions in a multiphase interstellar medium

We extend previous studies of the physics of interstellar cloud collisions by beginning an investigation of the role of magnetic fields through two-di mensional magnetohydrodynamical (MHD) numerical simulations. In particular, we study head-on collisions between equal mass, mildly supersonic, diffuse clouds similar to those in our previous study. Here we include a moderate magnetic field, corresponding to beta = p(g)/p(b) = 4 and two limiting fiel d geometries,with the field lines parallel (aligned) and perpendicular (tra nsverse) to the colliding cloud motion. We explore both adiabatic and radia tive (eta = tau(rad)/tau(coll) similar or equal to 0.38) cases, and we simu late collisions between clouds evolved through prior motion in the interclo ud medium. In addition to the collision of evolved identical clouds (symmet ric cases), Ive also study collisions of clouds that are initially identica l but have different evolutionary ages (asymmetric cases). Depending on the ir geometry, magnetic fields can significantly alter the outcome of the col lisions compared to the hydrodynamic (HD) case. (1) In the aligned case, ad iabatic collisions, like their HD counterparts, are very disruptive indepen dently of the symmetry. However, when radiative processes are taken into ac count, partial coalescence takes place even in the asymmetric case, unlike the HD calculations. (2) In the transverse case, the effects of the magneti c field are even more dramatic, with remarkable differences between unevolv ed and evolved clouds. Collisions between (initially adjacent) unevolved cl ouds are almost unaffected by magnetic fields. However, the interaction wit h the magnetized intercloud gas during precollision evolution produces a re gion of very high magnetic energy in front of the cloud. In collisions betw een evolved clouds with transverse held geometry, this region acts like a b umper, preventing direct contact between the clouds and eventually reversin g their motion. The elasticity, defined as the ratio of the final to the in itial kinetic energy of each cloud, is about 0.5-0.6 in the cases we consid ered. This behavior is found ia both adiabatic and radiative cases.