THE EVOLUTION OF THE NEUTRAL GAS IN PLANETARY-NEBULAE - THEORETICAL-MODELS

This paper theoretically models the emission expected from shells (gen eralized to include tori or clumps) expanding away from the hot centra l stars of PNe. We examine the effects of shocks, FUV (6 eV < h nu < 1 3.6 eV), and soft X rays (50 eV less than or similar to h nu less than or similar to 1 KeV) on the predominantly neutral gas and follow the time dependent chemistry for H-2, solving for the chemical and tempera ture structure and the emergent spectrum of the evolving shell. We con sider a large interval of values for the mass of the central star (fro m 0.6 to 0.836 M.) and for the shell properties, using its density and filling factor as free parameters. The calculations give the time dep endent physical and chemical properties of the shell (temperature, fra ctional abundances of HII, HI, H-2 and electrons), as well as the inte nsities of a number of lines of molecular hydrogen (H-2 v=1-0S(1); v=2 -1S(1); and v=0-0S(0), Si(1), S(2), S(3), and S(4)) Br gamma and the m etal lines CLI 158 mu m, OI 63 mu m, SiII 35 mu m, OI 6300 Angstrom, F eII 1.26 mu m, which can be compared to the observations and used to d etermine the physical parameters of the ejection process. We focus on the shell evolution after the star has achieved T- greater than or si milar to 30,000 K. If the column density in the shell is sufficiently high, a three-layered shell is produced with an inner HII region, a ce ntral HI region, and an outer H-2 region. In this case, we can identif y three phases in the evolution of the neutral shell. i) The early evo lution (T- similar to 30,000 K) is dominated by FUV photons, as the F UV photon luminosity Phi(FUV) of the central star peaks. The shell has a large column of warm H-2 and is very bright in all lines. The vibra tionally excited H-2 lines at 2 mu m are dominated by thermal emission from collisionally excited levels; the heating is predominantly by gr ain photoelectric heating and FUV pumping of H-2. ii) At somewhat late r times, as Phi(FUV) and gas density decline, the molecular gas become s cooler and the line intensity decreases rapidly. This is the only ph ase in which the emission of the v=1-0 H-2 lines can be dominated by f luorescence, and this fluorescent phase is present only in PNe with lo w-mass central stars. iii) At even later times, the star heats to T- > 100,000 K and soft X-rays heat and partially ionize the neutral gas well above the values determined by the FUV stellar radiation. The dur ation land presence) of these phases depends on the evolution with tim e of the stellar radiation field (i.e., on the mass of the central sta r), which is the main parameter that controls the PN evolution. For ex ample, we find that a standard M- = 0.6 M. central star produces phas e (i) from roughly 1000 to 5000 yrs, phase iii) from 5000 to 7000 yrs, and phase (iii) from 7000 yrs onward. PNe with high mass central star s reach high effective temperatures very quickly, and spend most of th eir life in the X-ray dominated phase. A M- = 0.836 M. case reaches p hase (iii) in roughly 1000 yrs. The decrease with time of the Hz line intensity (both in the near and mid-infrared) is less rapid than in PN e with low-mass central stars. Time-dependent Hz chemistry enhances ev en further the intensity of these lines. As a result, we find that mod els with high-mass central stars are the only cases which radiatively produce strong hydrogen molecular line intensities in old (large) PNe. For standard values of the parameters the emission in the vibrational ly excited Ha lines produced in the shock between the expanding shell and the precursor red giant wind is generally small compared to the PD R emission. However, for large red giant wind mass loss rates, (M)over dot(RG) greater than or similar to 10(-5) M. yr(-1), the shock emissi on can be significant. Therefore, strong H-2 2 mu m emission may also arise from shocks in old PNe.