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.
