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We present semi-analytic dynamical models for giant molecular clouds evolvingunder the influence of HII regions launched by newborn star clusters. Incontrast to previous work, we neither assume that clouds are in virial orenergetic equilibrium, nor do we ignore the effects of star formation feedback.The clouds, which we treat as spherical, can expand and contract homologously.Photoionization drives mass ejection; the recoil of cloud material both stirsturbulent motions and leads to an effective confining pressure. The balancebetween these effects and the decay of turbulent motions through isothermalshocks determines clouds' dynamical and energetic evolution. We find that forrealistic values of the rates of turbulent dissipation, photoevaporation, andenergy injection by HII regions, the massive clouds where most molecular gas inthe Galaxy resides live for a few crossing times, in good agreement with recentobservational estimates that large clouds in local group galaxies surviveroughly 20-30 Myr. During this time clouds remain close to equilibrium, withvirial parameters of 1-3 and column densities near 10^22 H atoms cm^-2, also inagreement with observed cloud properties. Over their lives they convert 5-10%of their mass into stars, after which point most clouds are destroyed when alarge HII region unbinds them. In contrast, small clouds like those found inthe solar neighborhood only survive ~1 crossing time before being destroyed.Comment: Accepted for publication in ApJ. 25 pages, 8 figures, emulateapj  forma

The Global Evolution of Giant Molecular Clouds. I. Model Formulation and Quasi‐Equilibrium Behavior