A photodissociation region (PDR) model grid in planetary nebula paradigm: dependency of C and O fine structure line emission and PDR thermal structure on central star radiation and nebular dust composition

Abstract

ABSTRACT We compute a grid of photodissociation region (PDR) models. The varying parameters of the grid include effective temperature (Teff) and luminosity (L) of the radiation source, density (nH) of the nebula, PDR depth from the nebular ionization front (ΔrPDR), and densities of polycyclic aromatic hydrocarbon (PAH) molecules and amorphous carbon (AC) dust. The paradigm, including the range of the parameters used to calculate the models, corresponds to planetary nebulae (PNe). We study the heating and cooling rates, and the variation of gas kinetic temperature (TK) along the PDR depth. We evaluate fluxes of fine structure lines: [O i] 63 $\mu$m, [C ii] 158 $\mu$m, and [C i] 609 $\mu$m and flux ratios: [O i] 146/63 $\mu$m, [O i] 63/[C ii] 158 $\mu$m, and [C i] 609/370 $\mu$m, and study their variation in the parameter space: Teff, L, and nH, for different composition of PAH and AC. We further study the distribution of [O i] 146/63 $\mu$m and [O i] 63/[C ii] 158 $\mu$m ratios in the parameter spaces of nH, ΔrPDR, Teff, and L. We take the observed [O i] 146/63 $\mu$m and [O i] 63/[C ii] 158 $\mu$m ratios for a number of PNe, and compare with these ratios obtained from our models. We verify that the correlation found between the [O i] 146/63 $\mu$m and [O i] 63/[C ii] 158 $\mu$m ratios observed in real PNe is similar to that found from our PN model grid.