Clumps and triggered star formation in ionised molecular clouds
  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.
