Dispersal of molecular clouds by ionising radiation
  The role of feedback from massive stars is believed to be a key element in
the evolution of molecular clouds. We use high-resolution 3D SPH simulations to
explore the dynamical effects of a single O7 star located at the centre of a
molecular cloud with mass 10^4M_sun and radius 6.4pc. The initial internal
structure of the cloud is characterised by its fractal dimension, D=2.0 - 2.8,
and its log-normal density PDF. (i) As regards star formation, in the short
term ionising feedback is positive, in the sense that star formation occurs
much more quickly in gas that is compressed by the high pressure of the ionised
gas. However, in the long term ionising feedback is negative, in the sense that
most of the cloud is dispersed with an outflow rate of up to ~0.01M_sun/yr, on
a timescale comparable with the sound-crossing time for the ionised gas
(~1-2Myr), and triggered star formation is therefore limited to a few percent
of the cloud's mass. (ii) As regards the morphology of the ionisation fronts
(IFs) bounding the HII region and the systematics of outflowing gas, we
distinguish two regimes. For low D<=2.2, the initial cloud is dominated by
large-scale structures, so the neutral gas tends to be swept up into a few
extended coherent shells, and the ionised gas blows out through a few large
holes between these shells; we term these HII regions "shell-dominated".
Conversely, for high D>=2.6, the initial cloud is dominated by small-scale
structures, and these are quickly overrun by the advancing IF, thereby
producing neutral pillars whilst the ionised gas blows out through a large
number of small holes between the pillars; we term these HII regions
"pillar-dominated". (iii) As regards the injection of bulk kinetic energy, by
~1Myr, the expansion of the HII region has delivered a rms velocity of ~6km/s;
this represents less than 0.1% of the total energy radiated by the O7 star.
