Clumps and triggered star formation in ionized molecular clouds

Abstract

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 effectof a single O7 star emitting photons at 1049 s-1 and located at the centre of a molecular cloud with mass 104 M 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, σso (with 0.36 ≤ σso ≤ 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. © 2013 The Authors.