Turbulent mixing in wind-blown HII regions

Abstract

We describe the process of turbulent mixing, occuring at the interface between the stellar wind bubble and the overlying, compressed HII layer. It is shown that hot electrons from the bubble can penetrate into the mixing region, where they deposit some fraction of their thermal energy. This first leads to an expansion of the gas at constant pressure, which soon becomes too inefficient to carry away the additional energy. Therefore pressure can build up in the mixing layer, forcing the gas to expand into the bubble. We also discuss the effect of the over-pressure on the HII shell and find that it is possible to punch a hole at the polar caps, where mixing is most efficient. This may lead to the formation of bipolar outflows. We apply our model to the wind-blown HII region NGC 6334(A), and show that it can explain the observed distortion of the bubble and the bipolar ionization cones.