TWO-FLUID MAGNETOHYDRODYNAMIC SIMULATIONS OF CONVERGING H I FLOWS IN THE INTERSTELLAR MEDIUM. I. METHODOLOGY AND BASIC RESULTS

We develop an unconditionally stable numerical method for solving the coupling between two fluids ( frictional forces/heatings, ionization, and recombination), which can adequately solve the evolution of a partially ionized medium from weak-coupling to strong-coupling regimes. By using two-dimensional two-fluid magnetohydrodynamical simulations based on this method, we investigate the dynamical condensation process of thermally unstable gas that is provided by the shock waves in a weakly ionized and magnetized interstellar medium. If we neglect the effect of magnetic field, it is known that condensation driven by thermal instability can generate high-density clouds whose physical condition corresponds to molecular clouds ( precursor of molecular clouds). In this paper we study the effect of magnetic field on the evolution of supersonic converging H I flows and focus on the case in which the orientation of magnetic field to converging flows is orthogonal. We show that the magnetic pressure gradient parallel to the flows prevents the formation of high-density and high-column density clouds, but instead generates fragmented, filamentary H I clouds. With this restricted geometry, magnetic field drastically diminishes the opportunity of fast molecular cloud formation directly from the warm neutral medium, in contrast to the case without magnetic field.