Modeling UV Radiation Feedback from Massive Stars. II. Dispersal of Star-forming Giant Molecular Clouds by Photoionization and Radiation Pressure

UV radiation feedback from young massive stars plays a key role in the evolution of giant molecular clouds (GMCs) by photoevaporating and ejecting the surrounding gas. We conduct a suite of radiation hydrodynamic simulations of star cluster formation in marginally-bound, turbulent GMCs, focusing on the effects of photoionization and radiation pressure on regulating the net star formation efficiency (SFE) and cloud lifetime. We find that the net SFE depends primarily on the initial gas surface density, $\Sigma_0$, such that the SFE increases from 4% to 51% as $\Sigma_0$ increases from $13\,M_{\odot}\,{\rm pc}^{-2}$ to $1300\,M_{\odot}\,{\rm pc}^{-2}$. Cloud destruction occurs within $2$-$10\,{\rm Myr}$ after the onset of radiation feedback, or within $0.6$-$4.1$ freefall times (increasing with $\Sigma_0$). Photoevaporation dominates the mass loss in massive, low surface-density clouds, but because most photons are absorbed in an ionization-bounded Str\"{o}mgren volume the photoevaporated gas fraction is proportional to the square root of the SFE. The measured momentum injection due to thermal and radiation pressure forces is proportional to $\Sigma_0^{-0.74}$, and the ejection of neutrals substantially contributes to the disruption of low-mass and/or high-surface density clouds. We present semi-analytic models for cloud dispersal mediated by photoevaporation and by dynamical mass ejection, and show that the predicted net SFE and mass loss efficiencies are consistent with the results of our numerical simulations.