Modes of Star Formation in Finite Molecular Clouds

We analytically investigate the modes of gravity-induced star formation possible in idealized finite molecular clouds where global collapse competes against both local Jeans instabilities and discontinuous edge instabilities. We examine these timescales for collapse in spheres, discs, and cylinders, with emphasis on the structure, size, and degree of internal perturbations required in order for local collapse to occur before global collapse. We find that internal, local collapse is more effective for the lower dimensional objects. Spheres and discs, if unsupported against global collapse, must either contain strong perturbations or must be unrealistically large in order for small density perturbations to collapse significantly faster than the entire cloud. We find, on the other hand, that filamentary geometry is the most favorable situation for the smallest perturbations to grow before global collapse overwhelms them and that filaments containing only a few Jeans masses and weak density perturbations can readily fragment. These idealized solutions are compared with simulations of star-forming regions in an attempt to delineate the role of global, local, and edge instabilities in determining the fragmentation properties of molecular clouds. The combined results are also discussed in the context of recent observations of Galactic molecular clouds.