Hydrodynamics of Cometary Compact H II Regions

We present numerical radiation-hydrodynamic simulations of cometary H II regions for a number of champagne flow and bow shock models. For the champagne flow models we study smooth density distributions with both steep and shallow gradients. We also consider cases in which the ionizing star has a strong stellar wind and cases in which the star additionally has a proper motion within the ambient density gradient. We find that our champagne flow plus stellar wind models have limb-brightened morphologies and kinematics that can see the line-of-sight velocities change sign twice between the head and tail of the cometary H II region, with respect to the rest frame velocity. Our bow shock models show that pressure gradients across and within the shell are very important for the dynamics and that simple analytic models assuming thin shells in ram pressure balance are wholly inadequate for describing the shape and kinematics of these objects at early times in their evolution. The dynamics of the gas behind the shock in the neutral material ahead of the ionization front in both champagne flow and bow shock type cometary H II regions is also discussed. We present simulated emission-measure maps and long-slit spectra of our results. Our numerical models are not tailored to any particular object, but comparison with observations from the literature shows that, in particular, the models combining density gradients and stellar winds are able to account for both the morphology and general radial velocity behavior of several observed cometary H II regions, such as the well-studied object G29.96-0.02.