NUMERICAL SIMULATIONS OF TURBULENT MOLECULAR CLOUDS REGULATED BY RADIATION FEEDBACK FORCES. I. STAR FORMATION RATE AND EFFICIENCY

Abstract

Radiation feedback from stellar clusters is expected to play a key role in setting the rate and efficiency of star formation in giant molecular clouds. To investigate how radiation forces influence realistic turbulent systems, we have conducted a series of numerical simulations employing the Hyperion radiation hydrodynamics solver, considering the regime that is optically thick to ultraviolet and optically thin to infrared radiation. Our model clouds cover initial surface densities between Sigma(cl),0 similar to 10-300 M-circle dot pc(-2), with varying initial turbulence. We follow them through turbulent, self-gravitating collapse, star cluster formation, and cloud dispersal by stellar radiation. All our models display a log-normal distribution of gas surface density Sigma; for an initial virial parameter alpha(vir,0) = 2, the log-normal standard deviation is sigma(ln) (Sigma) = 1-1.5 and the star formation rate coefficient epsilon(ff),((rho) over bar) = 0.3-0.5, both of which are sensitive to turbulence but not radiation feedback. The net star formation efficiency (SFE) epsilon(final) increases with Sigma(cl),(0) and decreases with alpha(vir,0). We interpret these results via a simple conceptual framework, whereby steady star formation increases the radiation force, such that local gas patches at successively higher Sigma become unbound. Based on this formalism (with fixed sigma(ln) (Sigma)), we provide an analytic upper bound on epsilon(final), which is in good agreement with our numerical results. The final SFE depends on the distribution of Eddington ratios in the cloud and is strongly increased by the turbulent compression of gas.