AN EVOLUTIONARY MODEL FOR COLLAPSING MOLECULAR CLOUDS AND THEIR STAR FORMATION ACTIVITY. II. MASS DEPENDENCE OF THE STAR FORMATION RATE

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 less than or similar to 10(4) M-circle dot) spend most of their evolution at low SFRs, but end their lives with a mini-burst, reaching a peak SFR similar to 10(4) M-circle dot Myr(-1), although their time-averaged SFR is only < SFR > similar to 10(2) M-circle dot Myr(-1). The corresponding efficiencies are SFEfinal less than or similar to 60% and < SFE > less than or similar to 1%. For more massive clouds (M-max less than or similar to 10(5) M-circle dot), 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 < SFE > approximate to 100(1 + M-max/1.4 x 10(5) M-circle dot)(1.68) M-circle dot Myr(-1) and < SFE > approximate to 0.03(M-max/2.5 x 10(5) M-circle dot)(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.