Clumps and triggered star formation in ionized molecular clouds

Infrared shells and bubbles are ubiquitous in the Galaxy and can generally be associated with H II 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) s(-1) and located at the centre of a molecular cloud with mass 10(4) M-circle dot and radius 6.4 pc; the internal structure of the cloud is characterized by its fractal dimension, D (with 2.0 <= D <= 2.8), and the variance of its (lognormal) density distribution, sigma(2)(O) (with 0.36 <= sigma(2)(O) <= 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 H II 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 H II 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.