BOW SHOCK-MODELS OF ULTRACOMPACT H-II REGIONS

We present models of ultracompact H II regions as the bow shocks formed by massive stars, with strong stellar winds, moving supersonically through molecular clouds. The morphologies, sizes, and brightnesses of observed objects match our models well. We provide plausible models for the ultracompact H II regions G12.21-0.1, G29.96-0.02, G34.26 + 0.15, and G43.89-0.78. To do this we begin by calculating the equilibrium shape of the wind-blown shell, assuming momentum conservation. Then we illuminate the shell with ionizing radiation from the central star, perform radiative transfer for free-free emission through the shell, and visualize the resulting object at various angles for comparison with radio continuum maps. Our model unifies most of the observed morphologies of ultracompact H II regions, excluding only those objects with spherical shells. Ram pressure confinement greatly lengthens the life of ultracompact H II regions, explaining the large number that exist in the Galaxy despite their low apparent kinematic ages.