High- and Low-Mass Star Forming Regions from Hierarchical Gravitational
  Fragmentation. High local Star Formation Rates with Low Global Efficiencies
  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. In this simulation, the velocity dispersions at
all scales are caused primarily by infall motions rather than by random
turbulence. 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. 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 find that the
statistical distributions of physical properties of the dense cores in the
region of massive collapse compare very well with those from a recent survey of
the massive star forming region in the Cygnus X molecular cloud. The star
formation efficiency per free-fall time (SFE_ff) of the high-mass SF clump is
low, ~0.04. This occurs because the clump is accreting mass at a high rate, not
because its specific SFR (SSFR) is low. This implies that a low value of the
SFE_ff does not necessarily imply a low SSFR, but may rather indicate a large
gas accretion rate. We suggest that a globally low SSFR at the GMC level can be
attained even if local star forming sites have much larger values of the SSFR
if star formation is a spatially intermittent process, so that most of the mass
in a GMC is not participating of the SF process at any given time.
