TIME-DEPENDENT IONIZATION CALCULATIONS OF GASEOUS NEBULAE

We developed a new computer code which is able to treat the time depen dent photo-and collisional ionization of gaseous nebulae together with the consistent energy balance for arbitrary numbers of elements, ions and line transitions. The intention of this paper is to describe the basic numerical methods and a few test calculations as being parts of full spherically symmetric radiation-gasdynamic computations in the li mit of a nonmoving gas (v(r) = 0). Exemplarily we discuss the evolutio n of ionization fronts around a suddenly switched-on/switched-off cent ral star with T-eff = 50 000 K and L = 6000 L.. Evidence is presented that the photon velocities must artificially be limited in time depen dent astrophysical calculations when the equation of radiative transfe r is not explicitely treated in a time dependent way. Furthermore, com putations with different spatial resolutions demonstrate that a carefu l choice of the grid resolution in ionization calculations is essentia l for the interpretation of observed line intensities.