Disruption of Molecular Clouds by Expansion of Dusty H II Regions
  Dynamical expansion of H II regions around star clusters plays a key role in
dispersing the surrounding dense gas and therefore in limiting the efficiency
of star formation in molecular clouds. We use a semi-analytic method and
numerical simulations to explore expansion of spherical dusty H II regions and
surrounding neutral shells and the resulting cloud disruption. Our model for
shell expansion adopts the static solutions of Draine (2011) for dusty H II
regions and considers the contact outward forces on the shell due to radiation
and thermal pressures as well as the inward gravity from the central star and
the shell itself. We show that the internal structure we adopt and the shell
evolution from the semi-analytic approach are in good agreement with the
results of numerical simulations. Strong radiation pressure in the interior
controls the shell expansion indirectly by enhancing the density and pressure
at the ionization front. We calculate the minimum star formation efficiency
$\epsilon_{min}$ required for cloud disruption as a function of the cloud's
total mass and mean surface density. Within the adopted spherical geometry, we
find that typical giant molecular clouds in normal disk galaxies have
$\epsilon_{min} \lesssim 10$%, with comparable gas and radiation pressure
effects on shell expansion. Massive cluster-forming clumps require a
significantly higher efficiency of $\epsilon_{min} \gtrsim 50$% for disruption,
produced mainly by radiation-driven expansion. The disruption time is typically
of the order of a free-fall timescale, suggesting that the cloud disruption
occurs rapidly once a sufficiently luminous H II region is formed. We also
discuss limitations of the spherical idealization.
