Molecular Cloud Evolution III. Accretion vs. stellar feedback
  We numerically investigate the effect of feedback from the ionizing radiation
heating from massive stars on the evolution of giant molecular clouds (GMCs)
and their star formation efficiency (SFE). We find that the star-forming
regions within the GMCs are invariably formed by gravitational contraction.
After an initial period of contraction, the collapsing clouds begin forming
stars, whose feedback evaporates part of the clouds' mass, opposing the
continuing accretion from the infalling gas. The competition of accretion
against dense gas consumption by star formation (SF) and evaporation by the
feedback, regulates the clouds' mass and energy balance, as well as their SFE.
We find that, in the presence of feedback, the clouds attain levels of the SFE
that are consistent at all times with observational determinations for regions
of comparable SF rates (SFRs). However, we observe that the dense gas mass is
larger in general in the presence of feedback, while the total (dense gas +
stars) is nearly insensitive to the presence of feedback, suggesting that the
total mass is determined by the accretion, while the feedback inhibits mainly
the conversion of dense gas to stars. The factor by which the SFE is reduced
upon the inclusion of feedback is a decreasing function of the cloud's mass,
for clouds of size ~ 10 pc. This naturally explains the larger observed SFEs of
massive-star forming regions. We also find that the clouds may attain a
pseudo-virialized state, with a value of the virial mass very similar to the
actual cloud mass. However, this state differs from true virialization in that
the clouds are the center of a large-scale collapse, continuously accreting
mass, rather than being equilibrium entities.
