Ionizing feedback from massive stars in massive clusters - II. Disruption of bound clusters by photoionization

We present a smoothed particle hydrodynamics parameter study of the dynamical effect of
photoionization from O-type stars on star-forming clouds of a range of masses and sizes during
the time window before supernovae explode. Our model clouds all have the same degree of
turbulent support initially, the ratio of turbulent kinetic energy to gravitational potential energy
being set to Ekin/|Epot| = 0.7. We allow the clouds to form stars and study the dynamical effects
of the ionizing radiation from the massive stars or clusters born within them. We find that
dense filamentary structures and accretion flows limit the quantities of gas that can be ionized,
particularly in the higher density clusters. More importantly, the higher escape velocities in
our more massive (106 M) clouds prevent the H II regions from sweeping up and expelling
significant quantities of gas, so that the most massive clouds are largely dynamically unaffected
by ionizing feedback. However, feedback has a profound effect on the lower density 104 and
105 M clouds in our study, creating vast evacuated bubbles and expelling tens of per cent of
the neutral gas in the 3-Myr time-scale before the first supernovae are expected to detonate,
resulting in clouds highly porous to both photons and supernova ejecta.