Clumps and triggered star formation in ionised molecular clouds

TL;DR

This study uses high-resolution 3D simulations to investigate how the internal structure of molecular clouds influences the morphology of ionised regions and the resulting star formation patterns around a single massive star.

Contribution

It provides new insights into how fractal cloud structures affect the morphology of HII regions and the resulting star formation, highlighting the role of fractal dimension in shaping stellar outcomes.

Findings

Low fractal dimension leads to shell-like HII regions and clustered star formation.

High fractal dimension results in pillar-like structures and more isolated star formation.

Star formation occurs earlier in low fractal dimension clouds with higher stellar masses.

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 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.