Star formation activity in Molecular Clouds: the role of geometry and initial virial parameter

Abstract We present a systematic study of the role of cloud geometry in regulating the star formation efficiency (SFE) of molecular clouds. Using three-dimensional isothermal hydrodynamical simulations, we evolve Jeans-unstable filaments with different aspect ratios and compare them with cubic boxes having the same initial properties. We find that, statistically, all filaments form stars within approximately one free-fall time (τff) computed from the initial mean gas density (ρ), τff∝ρ−1/2, rather than on the much longer global longitudinal collapse timescale of the full filament, τfil∝AR τff, where AR is the filament aspect ratio. At a fixed virial parameter and Jeans number, the SFE is systematically lower when the mass is distributed in a filamentary geometry than when it is distributed in a more compact configuration. This difference is due to the stronger gravitational focusing in compact geometries, which leads to a more rapid collapse and higher mass accretion rate towards the central regions. If filaments are interpreted as linear collections of three-dimensional ‘building-boxes’ with lengths equal to their width, the SFE of the whole filament is statistically similar to that of a single building-box. This implies that, for filaments with a given width, virial parameter, and local Jeans mass, the number of protostars increases approximately linearly with the total filament mass (or length) in the regime explored here. This result is consistent with observations of different local molecular clouds reported in the literature. Our results highlight the role of cloud geometry in regulating the star formation process.