BREAKING THE SOUND BARRIER IN RECOMBINATION FRONTS

We exploit a generic instability in the integration of steady, sonic, near-isothermal flows to find the complete transition diagram for reco mbination fronts (for a model system of equations). The instability re quires the integration of the flow equations for speeds between the is othermal and adiabatic sound speeds to be performed with particular ca re. As a result of this, the previous work of Newman & Axford on the s tructure of recombination fronts neglected an important class of solut ion, that of transonic fronts; our method is readily extensible to a m ore complete treatment of the ionization structure. Future papers will apply these results in models of the structure of ultracompact H II r egions.