We have presented the results obtained with a new hydrocode which also calculates at every time step the ionization structure of a nebula. The ionization of the hydrogen and helium is calculated time-dependently and the ionization structure of carbon, nitrogen, oxygen and neon is performed under a steady-state approximation with up to five stages of ionization. The code includes the diffuse radiation from the recombinations of hydrogen and helium in two different approximations, the OTS approximation and the outward approximation. The checks performed with a widely used steady-state photoionization code demonstrate that, despite the approximations made, the calculation of the ionization structure and the resultant temperature distribution are quite precise. We have reviewed the evolution of H II regions in a low constant density medium (the standard case) and we will use it as the reference frame for future calculations. The expansion of H II regions is characterized by a faint and narrow ring in the emission of 6300 and 5200 which becomes broader and of lower intensity when the region enters its recombination and reionization phases. The analysis of the line profiles has revealed 6584 as a good kinematical indicator even at low expansion velocities. The diagnostic diagrams of the models corresponding to both evolution and spatial sequences agree well with the empirical results of BPT.
Future work will include several improvements to the code incorporating the action of winds and supernova explosions and more physical processes. The evolution of a giant H II region powered by a massive star cluster will be the topic of our next communication.
© European Southern Observatory (ESO) 1998
Online publication: February 4, 1998