  Recent observations of molecular clouds show that dense filaments are the
sites of present-day star formation. Thus, it is necessary to understand the
filament formation process because these filaments provide the initial
condition for star formation. Theoretical research suggests that shock waves in
molecular clouds trigger filament formation. Since several different mechanisms
have been proposed for filament formation, the formation mechanism of the
observed star-forming filaments requires clarification. In the present study,
we perform a series of isothermal magnetohydrodynamics simulations of filament
formation. We focus on the influences of shock velocity and turbulence on the
formation mechanism and identified three different mechanisms for the filament
formation. The results indicate that when the shock is fast, at shock velocity
v_sh = 7 km/s, the gas flows driven by the curved shock wave create filaments
irrespective of the presence of turbulence and self-gravity. However, at a slow
shock velocity v_sh = 2.5 km/s, the compressive flow component involved in the
initial turbulence induces filament formation. When both the shock velocities
and turbulence are low, the self-gravity in the shock-compressed sheet becomes
important for filament formation. Moreover, we analyzed the line-mass
distribution of the filaments and showed that strong shock waves can naturally
create high-line-mass filaments such as those observed in the massive
star-forming regions in a short time. We conclude that the dominant filament
formation mode changes with the velocity of the shock wave triggering the
filament formation.
