Two-Fluid MHD Simulations of Converging HI Flows in the Interstellar Medium. I: Methodology and Basic Results

TL;DR

This paper introduces a new numerical method for simulating two-fluid magnetohydrodynamics in the interstellar medium, revealing how magnetic fields influence the formation and structure of HI clouds during shock-induced condensation.

Contribution

It develops an unconditionally stable two-fluid MHD simulation technique and investigates magnetic field effects on cloud formation in converging HI flows.

Findings

Magnetic pressure prevents high-density cloud formation in orthogonal magnetic fields.

Magnetic fields lead to fragmented, filamentary HI cloud structures.

Magnetic fields reduce the likelihood of rapid molecular cloud formation.

Abstract

We develop an unconditionally stable numerical method for solving the coupling between two fluids (frictional forces/heatings, ionization, and recombination), and investigate the dynamical condensation process of thermally unstable gas that is provided by the shock waves in a weakly ionized and magnetized interstellar medium by using two-dimensional two-fluid magnetohydrodynamical simulations. 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 HI 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 HI 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.