Two‐Fluid Magnetohydrodynamic Simulations of Converging H<scp>i</scp>Flows in the Interstellar Medium. I. Methodology and Basic Results

Abstract

We develop an unconditionally stable numerical method for solving the\ncoupling between two fluids (frictional forces/heatings, ionization, and\nrecombination), and investigate the dynamical condensation process of thermally\nunstable gas that is provided by the shock waves in a weakly ionized and\nmagnetized interstellar medium by using two-dimensional two-fluid\nmagnetohydrodynamical simulations. If we neglect the effect of magnetic field,\nit is known that condensation driven by thermal instability can generate high\ndensity clouds whose physical condition corresponds to molecular clouds\n(precursor of molecular clouds). In this paper, we study the effect of magnetic\nfield on the evolution of supersonic converging HI flows and focus on the case\nin which the orientation of magnetic field to converging flows is orthogonal.\nWe show that the magnetic pressure gradient parallel to the flows prevents the\nformation of high density and high column density clouds, but instead generates\nfragmented, filamentary HI clouds. With this restricted geometry, magnetic\nfield drastically diminishes the opportunity of fast molecular cloud formation\ndirectly from the warm neutral medium, in contrast to the case without magnetic\nfield.\n