  Infrared shells and bubbles are ubiquitous in the Galaxy and can generally be
associated with HII regions formed around young, massive stars. In this paper,
we use high-resolution 3D SPH simulations to explore the effect of a single O7
star emitting photons at 10^49 1/s and located at the centre of a molecular
cloud with mass 10^4 M_sun and radius 6.4 pc; the internal structure of the
cloud is characterised by its fractal dimension, D (with 2.0 <= D <= 2.8), and
the variance of its (log-normal) density distribution, sigma_0^2 (with 0.36 <=
sigma_0^2 <= 1.42). Our study focuses on the morphology of the swept-up cold
gas and the distribution and statistics of the resulting star formation. If the
fractal dimension is low, the border of the HII region is dominated by extended
shell-like structures, and these break up into a small number of massive
high-density clumps which then spawn star clusters; star formation occurs
relatively quickly, and delivers somewhat higher stellar masses. Conversely, if
the fractal dimension is high, the border of the HII region is dominated by a
large number of pillars and cometary globules, which contain compact dense
clumps and tend to spawn single stars or individual multiple systems; star
formation occurs later, the stellar masses are somewhat lower, and the stars
are more widely distributed.
