2001MNRAS.323..343L

We have obtained 43-198µm far-infrared (IR) spectra for a sample of 51 Galactic planetary nebulae (PN) and protoplanetary nebulae (PPN), using the Long Wavelength Spectrometer (LWS) on board the Infrared Space Observatory (ISO). Spectra were also obtained of the former PN candidate Lo 14. The spectra yield fluxes for the fine-structure lines [Nii] 122µm, [Niii] 57µm and [Oiii] 52 and 88µm emitted in the ionized regions and the [Oi] 63- and 146-µm and [Cii] 158-µm lines from the photodissociation regions (PDRs), which have been used to determine electron densities and ionic abundances for the ionized regions and densities, temperatures and gas masses for the PDRs. The strong [Niii] and [Oiii] emission lines detected in the LWS spectrum taken centred on Lo 14 could be associated with the nearby strong radio and infrared source G 331.5-0.1.

We find that the electron densities yielded by the [Oiii] 88µm/52µm doublet ratio are systematically lower than those derived from the optical [Ariv] λ4740/λ4711 and [Cliii] λ5537/λ5517 doublet ratios, which have much higher critical densities than the 52- and 88-µm lines, suggesting the presence of density inhomogeneities in the nebulae. Ionic abundances, N⁺/H⁺, N²⁺/H⁺ and O²⁺/H⁺, as well as the N²⁺/O²⁺ abundance ratio, which provides a good approximation to the N/O elemental abundance ratio, are derived. Although ionic abundances relative to H⁺ deduced from the far-IR fine-structure lines are sensitive to the adopted electron density and the presence of density inhomogeneities, the strong dependence on the nebular physical conditions is largely cancelled out when N²⁺/O²⁺ is calculated from the 57µm/(52µm+88µm) flux ratio, owing to the similarity of the critical densities of the lines involved.

The temperatures and densities of the PDRs around 24 PN have been determined from the observed [Oi] and [Cii] line intensity ratios. Except for a few objects, the deduced temperatures fall between 200 and 500K, peaking around 250K. The densities of the PDRs vary from 10⁴-10⁵cm^–3, reaching 3x10⁵cm^–3 in some young compact PN. With a derived temperature of 1600K and a density of 10⁵cm^–3, the PDR of NGC 7027 is one of the warmest and at the same time one of the densest amongst the nebulae studied. For most of the PN studied, the [Cii]-emitting regions contain only modest amounts of material, with gas masses ≲0.1M_☉. Exceptional large PDR masses are found for a few nebulae, including NGC 7027, the bipolar nebulae M2-9 and NGC 6302, the young dense planetary nebulae BD+30°3639, IC 418 and NGC 5315, and the old, probably recombining, nebulae IC 4406 and NGC 6072.