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-dimensional 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 β = p_g/p_b = 4, and two limiting field geometries, with the field lines parallel (aligned) and perpendicular (transverse) to the colliding cloud motion. We explore both adiabatic and radiative (η = τ_rad/τ_coll ≃ 0.38) cases, and we simulate collisions between clouds evolved through prior motion in the intercloud medium. In addition to the collision of evolved identical clouds (symmetric cases), we also study collisions of clouds that are initially identical but have different evolutionary ages (asymmetric cases). Depending on their geometry, magnetic fields can significantly alter the outcome of the collisions compared to the hydrodynamic (HD) case. (1) In the aligned case, adiabatic collisions, like their HD counterparts, are very disruptive independently of the symmetry. However, when radiative processes are taken into account, partial coalescence takes place even in the asymmetric case, unlike the HD calculations. (2) In the transverse case, the effects of the magnetic field are even more dramatic, with remarkable differences between unevolved and evolved clouds. Collisions between (initially adjacent) unevolved clouds are almost unaffected by magnetic fields. However, the interaction with the magnetized intercloud gas during precollision evolution produces a region of very high magnetic energy in front of the cloud. In collisions between evolved clouds with transverse field geometry, this region acts like a bumper, preventing direct contact between the clouds and eventually reversing their motion. The elasticity, defined as the ratio of the final to the initial kinetic energy of each cloud, is about 0.5-0.6 in the cases we considered. This behavior is found in both adiabatic and radiative cases.