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 emitting ionising photons at 10 ^ { 49 } { s } ^ { -1 } and located at the centre of a molecular cloud with mass 10 ^ { 4 } { M } _ { { } _ { \odot } } and radius 6.4 { pc } ; we also perform comparison simulations in which the ionising star is removed .
The initial internal structure of the cloud is characterised by its fractal dimension , which we vary between { \cal D } = 2.0 and { \cal D } = 2.8 , and the roughly constant standard deviation , \sigma _ { { } _ { 10 } } = 0.38 , of 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 ( than in the control simulations ) , 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 \sim 10 ^ { -2 } { M } _ { \odot } { yr } ^ { -1 } , on a timescale comparable with the sound-crossing time for the ionised gas ( \sim 1 { to } 2 { Myr } ) , and triggered star formation is therefore limited to a few percent of the cloud ’ s mass .
We will describe in greater detail the statistics of the triggered star formation in a companion paper .
( ii ) As regards the morphology of the ionisation fronts ( IFs ) bounding the H ii region and the systematics of outflowing gas , we distinguish two regimes .
For low { \cal D } \la 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 H ii regions shell-dominated .
Conversely , for high { \cal D } \ga 2.6 , the initial cloud is dominated by small-scale structures , and these are quickly overrun by the advancing IF , thereby producing neutral pillars protruding into the H ii region , whilst the ionised gas blows out through a large number of small holes between the pillars ; we term these H ii regions pillar-dominated .
( iii ) As regards the injection of bulk kinetic energy , by \sim 1 { Myr } , the expansion of the H ii has delivered a mass-weighted root-mean-square velocity of \sim 6 { km } { s } ^ { -1 } ; this represents less than 0.1 \% of the total energy radiated by the O7 star .