An evolutionary model for collapsing molecular clouds and their star formation activity. II. Mass dependence of the star formation rate.
ZAMORA-AVILES M. and VAZQUEZ-SEMADENI E.
Abstract (from CDS):
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 (Mmax ≲ 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 SFEfinal ≲ 60% and < SFE > ≲ 1%. For more massive clouds (Mmax ≳ 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 + Mmax/1.4x105 M☉)1.68 M☉M/yr and < SFE > ~ 0.03(Mmax/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.
galaxies: star formation - gravitation - H II regions - ISM: clouds - ISM: kinematics and dynamics - radiation: dynamics - stars: evolution - stars: formation - stars: massive - turbulence