Photoionized stellar wind bubbles in a cloudy medium

Abstract

The evolution of a stellar wind bubble in a cloudy medium of 1-10/cu cm mean density is investigated, allowing for the effects of the star's ionizing radiation. The dynamics of the bubble are governed by the effects of photoionization: weak winds produce bubbles which are confined by the pressure of the H II region, whereas strong winds produce bubbles whose size is determined by the photoevaporation and displacement of the clouds. Nebulae around massive stars begin with wind-blown shells, evolve into amorphous H II regions, then ring-like H II regions, then nebulae with stellar ejecta, and finally are swept up by a blast wave when the star explodes as a supernova. Part or all of the ring-like phase may be too faint to observe. Expanding H II shells with masses of the order of 10,000 solar masses are predicted around massive stars, and may account for the shells seen around the supernova remnants HB 21 and W44.