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Two-Fluid MHD Simulations of Converging Hi Flows in the Interstellar Medium. II: Are Molecular Clouds Generated Directly from Warm Neutral Medium?

Formation of interstellar clouds as a consequence of thermal instability is studied using two-dimensional two-fluid magnetohydrodynamic simulations. We consider the situation of converging, supersonic flows of warm neutral medium in the interstellar medium that generate a shocked slab of thermally unstable gas in which clouds form. We found, as speculated in paper I, that in the shocked slab magnetic pressure dominates thermal pressure and the thermal instability grows in the isochorically cooling, thermally unstable slab that leads formation of HI clouds whose number density is typically n < 100 cm^-3, even if the angle between magnetic field and converging flows is small. We also found that even if there is a large dispersion of magnetic field, evolution of the shocked slab is essentially determined by the angle between the mean magnetic field and converging flows. Thus, the direct formation of molecular clouds by piling up warm neutral medium does not seem a typical molecular cloud formation process, unless the direction of supersonic converging flows is biased to the orientation of mean magnetic field by some mechanism. However, when the angle is small, the HI shell generated as a result of converging flows is massive and possibly evolves into molecular clouds, provided gas in the massive HI shell is piled up again along the magnetic field line. We expect that another subsequent shock wave can pile up again the gas of the massive shell and produce a larger cloud. We thus emphasize the importance of multiple episodes of converging flows, as a typical formation process of molecular clouds.