Photoionisation Feedback in a Self-Gravitating, Magnetised, Turbulent Cloud

TL;DR

This study develops analytic models and conducts simulations to understand how UV photoionisation from massive stars influences the expansion of HII regions in turbulent, self-gravitating molecular clouds, revealing conditions for trapping and feedback effects.

Contribution

It introduces new analytic models predicting HII region behavior in turbulent clouds and compares them with detailed radiative magnetohydrodynamic simulations, highlighting cloud trapping and feedback mechanisms.

Findings

HII region expansion can be predicted by density and velocity structure.

Cloud trapping occurs when the ionisation front stalls before reaching the edge.

UV radiation generally suppresses star formation by evaporating unstable gas.

Abstract

We present a new set of analytic models for the expansion of HII regions powered by UV photoionisation from massive stars and compare them to a new suite of radiative magnetohydrodynamic simulations of turbulent, self-gravitating molecular clouds. To perform these simulations we use the Eulerian adaptive mesh magnetohydrodynamics code RAMSES-RT, including radiative transfer of UV photons. Our analytic models successfully predict the global behaviour of the HII region provided the density and velocity structure of the cloud is known. We give estimates for the HII region behaviour based on a power law fit to the density field assuming that the system is virialised. We give a radius at which the ionisation front should stop expanding ("stall"). If this radius is smaller than the distance to the edge of the cloud, the HII region will be trapped by the cloud. This effect is more severe in collapsing clouds than in virialised clouds, since the density in the former increases dramatically over time, with much larger photon emission rates needed for the HII region to escape a collapsing cloud. We also measure the response of Jeans unstable gas to the HII regions to predict the impact of UV radiation on star formation in the cloud. We find that the mass in unstable gas can be explained by a model in which the clouds are evaporated by UV photons, suggesting that the net feedback on star formation should be negative