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High- and Low-Mass Star Forming Regions from Hierarchical Gravitational Fragmentation

(Abridged) We investigate the properties of "star forming regions" in a previously published numerical simulation of molecular cloud formation out of compressive motions in the warm neutral atomic interstellar medium, neglecting magnetic fields and stellar feedback. We study the properties (density, total gas+stars mass, stellar mass, velocity dispersion, and star formation rate) of the cloud hosting the first local, isolated "star formation" event in the simulation and compare them with those of the cloud formed by a later central, global collapse event. In this simulation, the velocity dispersions at all scales are caused primarily by infall motions rather than by random turbulence. We suggest that the small-scale, isolated collapse may be representative of low- to intermediate-mass star-forming regions, while the large-scale, massive one may be representative of massive star forming regions. We also obtain the statistical distributions of the physical properties of the dense cores appearing in the central region of massive collapse, and compare them with those from a recent survey of the massive star forming region in the Cygnus X molecular cloud by Motte et al. We find that the observed and simulated distributions are in general very similar, reinforcing the suggestion that massive star forming regions are sites of global, large-scale collapse. This suggestion contrasts with the common notion that these regions are in rough virial equilibrium, supported by strong turbulence.