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.
