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 .