2014ApJ...793...84Z

We discuss the evolution and dependence on cloud mass of the star formation rate (SFR) and efficiency (SFE) of star-forming molecular clouds (MCs) within the scenario that clouds are undergoing global collapse and that the SFR is controlled by ionization feedback. We find that low-mass clouds (M_max ≲ 10⁴ M_☉) spend most of their evolution at low SFRs, but end their lives with a mini-burst, reaching a peak SFR ∼10⁴ M_☉M/yr, although their time-averaged SFR is only < SFR > ∼ 10² M_☉M/yr. The corresponding efficiencies are SFE_final ≲ 60% and < SFE > ≲ 1%. For more massive clouds (M_max ≳ 10⁵ M_☉), the SFR first increases and then reaches a plateau because the clouds are influenced by stellar feedback since earlier in their evolution. As a function of cloud mass, < SFR > and < SFE > are well represented by the fits < SFR > ~ 100(1 + M_max/1.4x10⁵ M_☉)^1.68 M_☉M/yr and < SFE > ~ 0.03(M_max/2.5x10⁵ M_☉)^0.33, respectively. Moreover, the SFR of our model clouds follows closely the SFR-dense gas mass relation recently found by Lada et al. during the epoch when their instantaneous SFEs are comparable to those of the clouds considered by those authors. Collectively, a Monte Carlo integration of the model-predicted SFR(M) over a Galactic giant molecular cloud mass spectrum yields values for the total Galactic SFR that are within half an order of magnitude of the relation obtained by Gao & Solomon. Our results support the scenario that star-forming MCs may be in global gravitational collapse and that the low observed values of the SFR and SFE are a result of the interruption of each SF episode, caused primarily by the ionizing feedback from massive stars.