Stellar winds are an essential ingredient in understanding the dynamical evolution of HII regions and in interpreting observations carried out in different spectral domains. The kinematic structure produced by the expansion of the bubble of shocked stellar wind after blowing out of the parental molecular cloud is sensibly different from that obtained by considering only the photoionizing flux of the central star, leading to a much larger volume affected by the expansion of the wind-driven bubble. Theoretical maps of X-ray emission, free-free continuum emission at long wavelengths, and line-to-continuum ratios at a fixed frequency have been presented and interpreted in terms of the dynamical evolution of the gas, followed by means of 2-D axisymmetric numerical simulations.
The main differences between the cases with and without wind take place in the expansion of the hot bubble in the intercloud medium. Features of the classical champagne model, such as the expansion of the compact component in the parental molecular cloud and the outflow of dense, ionized gas (the champagne flow) into the intercloud medium, are not substantially changed when including the stellar wind. The wind has an indirect influence in the destruction of the molecular cloud by keeping a higher pressure inside the compact component than in the windless case. However, this higher pressure produces effects going in different directions, as explained in Sect. 3.1.5, and although the net result in the cases explored here is a somewhat faster rate of photoionization of fresh dense cloud material, the differences introduced by the stellar wind are not dramatic as far as the destruction of the cloud is concerned. On the other hand, as discussed in 3.2.3 and shown in Fig. 14, there is almost no difference in the integrated line profiles between the cases with and without wind. This is a direct consequence of the fact that most of the high density ionized gas is contained in the compact component and the champagne flow.
We have carried out an obviously very limited exploration of possible parameters which define the initial conditions of the problem. Even so, it is clear from our results that dramatic variations arise in the dynamical evolution from changing the energetic output characteristics of the exciting star, its distance to the cloud-intercloud boundary, or the physical ingredients included in the simulations, such as thermal conduction. Moreover, the simulated maps presented here show the influence of factors such as the orientation of the HII region with respect to the line of sight or the extinction inside the HII region itself. This illustrates the importance of a proper choice of the input parameters and the viewing geometry when interpreting real observations of HII regions.
© European Southern Observatory (ESO) 1997
Online publication: April 8, 1998