H II regions: Witnesses to massive star formation

TL;DR

This paper presents the first 3D simulation of massive star formation including radiation effects, revealing how H II regions evolve and influence accretion, leading to high-mass star formation through competitive accretion and fragmentation.

Contribution

It introduces a novel 3D simulation model that incorporates both non-ionizing and ionizing radiation in massive star formation, explaining observed H II region behaviors and the mechanisms setting stellar mass.

Findings

H II regions fluctuate between trapped and extended states, matching observations.

Expanding H II regions drive bipolar outflows characteristic of high-mass stars.

Final stellar mass is determined by fragmentation-induced starvation, not hot gas pressure.

Abstract

We describe the first three-dimensional simulation of the gravitational collapse of a massive, rotating molecular cloud that includes heating by both non-ionizing and ionizing radiation. We find that as the first protostars gain sufficient mass to ionize the accretion flow, their H II regions are initially gravitationally trapped, but soon begin to rapidly fluctuate between trapped and extended states, in agreement with observations. Over time, the same ultracompact H II region can expand anisotropically, contract again, and take on any of the observed morphological classes. In their extended phases, expanding H II regions drive bipolar neutral outflows characteristic of high-mass star formation. The total lifetime of H II regions is given by the global accretion timescale, rather than their short internal sound-crossing time. The pressure of the hot, ionized gas does not terminate accretion. Instead the final stellar mass is set by fragmentation-induced starvation. Local gravitational instabilities in the accretion flow lead to the build-up of a small cluster of stars, all with relatively high masses due to heating from accretion radiation. These companions subsequently compete with the initial high-mass star for the same common gas reservoir and limit its mass growth. Our findings show that the most significant differences between the formation of low-mass and high-mass stars are all explained as the result of rapid accretion within a dense, gravitationally unstable, ionized flow.