ISO LWS observations of planetary nebula fine-structure lines

Abstract

We have obtained 43–198 μm 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 [N ii] 122 μm, [N iii] 57 μm and [O iii] 52 and 88 μm emitted in the ionized regions and the [O i] 63‐ and 146‐μm and [C ii] 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 [N iii] and [O iii] 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 [O iii] 88 μm/52 μm doublet ratio are systematically lower than those derived from the optical [Ar iv] λ4740/λ4711 and [Cl iii] 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 57 μm/(52 μm+88 μm) 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 [O i] and [C ii] line intensity ratios. Except for a few objects, the deduced temperatures fall between 200 and 500 K, peaking around 250 K. The densities of the PDRs vary from 104–105 cm−3, reaching 3×105 cm−3 in some young compact PN. With a derived temperature of 1600 K and a density of 105 cm−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 [C ii]‐emitting regions contain only modest amounts of material, with gas masses ≲0.1 M⊙. 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.

Far-infrared metallicity diagnostics: application to local ultraluminous infrared galaxies

“…The [O iii] to [N iii] ratios have been already identified as far-IR metallicity tracers (e.g., Liu et al 2001;Nagao et al 2011), although they are strongly dependent on the electron density (see left and middle panels of Figure 5). However, for a fixed metallicity, the density dependence of the Figure 1, but for the left and middle panels, we group the models by their density and the shaded area is given by the ionization parameter dependence, while for the right panel, we group the models by their ionization parameter and the shaded area is given by the density dependence.…”

IFU spectroscopy of southern planetary nebulae IV: a physical model for IC 418

“…Although we have not specifically modelled the UV and IR spectrum of IC 418, nonetheless it is of interest to see how well the model performs in reproducing the de-reddened UV and IR line intensities given by Pottasch et al (2004) and by Liu et al (2001), who used the LWS on the ISO observatory to measure the longer wavelength (43-198µm) lines. The comparison between the model and these observations is given in Table 7.…”

FIFI-LS: The Field-Imaging Far-Infrared Line Spectrometer on SOFIA

“…In order to check the calibration,°uxes measured with FIFI-LS are compared with ISO LWS measurements from Liu et al (2001). To account for the uncertainty of telluric correction, 50% is added to or subtracted from the standard zenith values of water vapor used by ATRAN (e.g.…”