The hydrodynamics and ionization structure of gaseous nebulae I. The time-dependent solution

We present a one-dimensional numerical scheme that allows for the simultaneous solution of the hydrodynamic equations coupled with energy and ionization equations. The code accounts for the time-dependent ionization of H, He, and He+ and for radiative cooling derived from collisional excitation of C, N, O and Ne, whose ionization structures with up to five stages of ionization are calculated under a steady approximation at every time step. Several other physical processes such as collisional ionization, charge-exchange reactions and bremsstrahlung have been taken into consideration. Some assumptions and approximations have been made in order to minimize the computational time. However, the comparison with the results from other steady-state photoionization codes indicates a very precise calculation of the ionization structure. The computational method is meant to be general and allows for a wide variety of ionizing sources (e.g. a black body, a stellar atmosphere model and a power law) as well as their possible evolution in time. The code is supplemented with the necessary tools to calculate the synthesized spectrum from the numerical models, including the most important lines of the elements mentioned in order to compare directly with the observations. In this paper the code is described and the first calculations for the standard evolution of H It regions are shown. The results generally agree with previous works but in addition they go to greater depth showing some interesting aspects of the evolution of gaseous nebulae.