How D-type HII region expansion depends on numerical resolution

We investigate the resolution dependence of HII regions expanding past their Str"{o}mgren spheres. We find that their structure and size, and the radial momentum that they attain at a given time, is in good agreement with analytical expectations if the Str"{o}mgren radius is resolved with $dr \leq 0.3,R_{\rm st}$. If this is not satisfied, the radial momentum may be over- or under-estimated by factors up to 10 or more. Our work has significance for the amount of radial momentum that a HII region can impart to the ambient medium in numerical simulations, and thus on the relative importance of ionizing feedback from massive stars.