# When H II Regions are Complicated: Considering Perturbations from Winds, Radiation Pressure, and Other Effects

## TL;DR

This paper develops algebraic models to analyze the dynamics of H II regions considering winds, radiation pressure, gravity, and photon breakout, highlighting photoionisation as the dominant feedback mechanism in molecular clouds.

## Contribution

It introduces algebraic models that incorporate multiple physical effects to evaluate their significance in H II region evolution, emphasizing the dominant role of photoionisation feedback.

## Key findings

- Photoionisation feedback drives outflows in molecular clouds.
- Winds and radiation pressure contribute about 10% to H II region dynamics.
- Effects of winds and radiation are most significant at high densities and close to the star.

## Abstract

We explore to what extent simple algebraic models can be used to describe H II regions when winds, radiation pressure, gravity and photon breakout are included. We a) develop algebraic models to describe the expansion of photoionised H II regions under the influence of gravity and accretion in power-law density fields with $\rho \propto r^{-w}$, b) determine when terms describing winds, radiation pressure, gravity and photon breakout become significant enough to affect the dynamics of the H II region where $w=2$, and c) solve these expressions for a set of physically-motivated conditions. We find that photoionisation feedback from massive stars is the principal mode of feedback on molecular cloud scales, driving accelerating outflows from molecular clouds in cases where the peaked density structure around young massive stars is considered at radii between $\sim$0.1 and 10-100 pc. Under a large range of conditions the effect of winds and radiation on the dynamics of H II regions is around 10% of the contribution from photoionisation. The effect of winds and radiation pressure are most important at high densities, either close to the star or in very dense clouds such as those in the Central Molecular Zone of the Milky Way. Out to $\sim$0.1 pc they are the principal drivers of the H II region. Lower metallicities make the relative effect of photoionisation even stronger as the ionised gas temperature is higher.