The Early Stage of Molecular Cloud Formation by Compression of Two-phase
  Atomic Gases
  We investigate the formation of molecular clouds from atomic gas by using
three-dimensional magnetohydrodynamic simulations, including non-equilibrium
chemical reactions and heating/cooling processes. We consider super-Alfv\'enic
head-on colliding flows of atomic gas possessing the two-phase structure that
consists of HI clouds and surrounding warm diffuse gas. We examine how the
formation of molecular clouds depends on the angle $\theta$ between the
upstream flow and the mean magnetic field. We find that there is a critical
angle $\theta_\mathrm{cr}$ above which the shock-amplified magnetic field
controls the post-shock gas dynamics. If the atomic gas is compressed almost
along the mean magnetic field ($\theta\ll\theta_\mathrm{cr}$), super-Alfv\'enic
anisotropic turbulence is maintained by the accretion of the highly
inhomogeneous upstream atomic gas. As a result, a greatly extended
turbulence-dominated post-shock layer is generated. Around $\theta\sim
\theta_\mathrm{cr}$, the shock-amplified magnetic field weakens the post-shock
turbulence, leading to a dense post-shock layer. For $\theta\gg
\theta_\mathrm{cr}$, the strong magnetic pressure suppresses the formation of
cold dense clouds. Efficient molecular cloud formation is expected if $\theta$
is less than a few times $\theta_\mathrm{cr}$. Developing an analytic model and
performing a parameter survey, we obtain an analytic formula for the critical
angle as a function of the mean density, collision speed, and field strength of
the upstream atomic gas. The critical angle is found to be less than $\sim
15^\circ$ as long as the field strength is larger than $1~\mu$G, indicating
that the probability of occurrence of compression with
$\theta<\theta_\mathrm{cr}$ is limited if shock waves come from various
directions.
