Clumpy ultracompact H II regions – II . Cores , spheres and shells from subsonic flows

We have modelled ultracompact H ii regions (UCHiiR) in terms of steady subsonic ionized flows in a clumpy medium. Mass loss from neutral clumps allows the regions to be long-lived. We examine the form of global flows for different dependences of the volume mass injection rate, ˙ q, on radius and Mach number, and describe the solutions in detail. We find that three observed UCHiiR morphologies are reproduced with these models. Mach number independent flows that include a radial variation can give centre-brightened core–halo morphologies. Mach number dependent flows reproduce naturally the uniform UCHiiR morphology. In a hybrid model, including subsonic and supersonic flows, we allow a supersonic wind to shock in the ionized region. The ionized subsonic gas has a high density and so dominates the emission. The shell produced has a velocity structure very different from that of fully supersonic models. Several morphologies of spherical UCHiiR can be understood in terms of these various models; however, kinematic data are crucial as a discriminant between them.