How D-type H II region expansion depends on numerical resolution

We investigate the resolution dependence of H II regions expanding past their Strömgren 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 Strömgren radius is resolved with dr≤0.3Rst⁠. If this is not satisfied, the radial momentum may be over- or underestimated by factors up to 10 or more. Our work has significance for the amount of radial momentum that a H II region can impart to the ambient medium in numerical simulations, and thus, on the relative importance of ionizing feedback from massive stars.