Classification of Filament Formation Mechanisms

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 this chapter, we perform isothermal MHD simulations of filament formation. We focus on the influences of shock velocity on the formation mechanism and identify three mechanisms for filament formation. The results indicate that when the shock is fast, at shock velocity \(\textrm{v}_{\textrm{sh}} \simeq 7 \textrm{km s}^{-1}\) , the gas flows driven by the curved shock wave create filaments irrespective of turbulence and self-gravity. However, at a slow shock velocity \(\textrm{v}_{\textrm{sh}} \simeq 2.5 \textrm{km s}^{-1}\) , the compressive flow component involved in the initial turbulence induces filament formation. We conclude that the dominant filament formation mode changes with the velocity of the shock wave triggering the filament formation.