Massive stars in massive clusters II: Disruption of bound clusters by
  photoionization
  We present an SPH 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 $E_{\rm kin}/|E_{\rm
pot}|$=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
(10$^{6}$M$_{\odot}$) clouds prevent the HII 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 10$^{4}$ and 10$^{5}$M$_{\odot}$ clouds
in our study, creating vast evacuated bubbles and expelling tens of percent of
the neutral gas in the 3Myr timescale before the first supernovae are expected
to detonate, resulting in clouds highly porous to both photons and supernova
ejecta.
