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 and compare them with those of the cloud formed by the 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 collapses may be representative of low- to intermediate-mass star-forming regions , with gas masses ( M _ { gas } ) of hundreds of solar masses , velocity dispersions \sigma _ { v } \sim 0.7 { ~ { } km~ { } s } ^ { -1 } , and star formation rates ( SFRs ) \sim 3 \times 10 ^ { -5 } M _ { \odot } { yr } ^ { -1 } , while the large-scale , massive ones may be representative of massive star forming regions , with M _ { gas } of thousands of solar masses , \sigma _ { v } \sim a few { ~ { } km~ { } s } ^ { -1 } , and SFRs \sim 3 \times 10 ^ { -4 } M _ { \odot } { yr } ^ { -1 } .
We also compare the statistical distributions of the physical properties of the dense cores appearing in the central region of massive collapse with those from a recent survey of the massive star forming region in the Cygnus X molecular cloud , finding that the observed and simulated distributions are in general very similar .

However , we find that the star formation efficiency per free-fall time ( SFE _ { ff } ) of the high mass region , similarly to that of OMC-1 , is low , \sim 0.04 .
In the simulated cloud , this is not a consequence of a “ slow ” SFR in a nearly hydrostatic cloud supported by turbulence , but rather of the region accreting mass at a high rate .
Thus , we find that measuring a low SFE _ { ff } may be incorrectly interpreted as implying a lifetime much longer than the core ’ s local free-fall time , and an SFR much slower than that given by the free-fall rate , if the accretion is not accounted for .
We suggest that , rather than requiring a low value of the SFE _ { ff } everywhere in the Galaxy , attaining a globally low specific SFR requires star formation to be a spatially intermittent process , so that most of the mass in a GMC is not participating of the SF process at any given time .
Locally , the specific SFR of a star-forming region can be much larger than the global GMC ’ s average .