The FUSE Spectrum of the Planetary Nebula SwSt 1: Evidence for Inhomogeneities in the Gas and Dust

We present Far Ultraviolet Spectroscopic Explorer (FUSE) observations of the young, compact planetary nebula (PN) SwSt 1 along the line of sight to its central star HD 167362. We detect circumstellar absorption lines from several species against the continuum of the central star. The physical parameters of the nebula derived from the FUSE data differ significantly from those found from emission lines. We derive an electron density ne = 8800 cm-3 from the column density ratio of the excited S III fine-structure levels, which is at least a factor of 3 lower than all prior estimates. The gaseous iron abundance derived from the UV lines is quite high ([Fe/S] = -0.35 ± 0.12), which implies that iron is not significantly depleted into dust. In contrast, optical and near-infrared emission lines indicate that Fe is more strongly depleted: [Fe/H] = -1.64 ± 0.24 and [Fe/S] = -1.15 ± 0.33. We do not detect nebular H2 absorption, to a limit N(H2) < 7 × 1014 cm-2, at least 4 orders of magnitude lower than the column density estimated from infrared H2 emission lines. Taken together, the lack of H2 absorption, low ne, and high gaseous Fe abundance derived from the FUSE spectrum provide strong evidence that dense structures (which can shield molecules and dust from the destructive effects of energetic stellar photons) are not present along the line of sight to the central star. On the other hand, there is substantial evidence for dust, molecular material, and dense gas elsewhere in SwSt 1. Therefore, we conclude that the nebula must have an inhomogeneous structure. We detect nebular absorption at 1040.94 and 1041.69 A from the two excited fine-structure levels of neutral oxygen. These levels give rise to far-infrared emission lines at 63 and 145 μm, which are often used to infer gas properties, particularly temperature, under the assumption that they are collisionally excited. We find that the O I fine-structure levels in SwSt 1 have an inverted population ratio. This requires a nonthermal excitation mechanism, which we identify as fluorescent excitation by the stellar continuum. To the extent that fluorescence affects the level populations, the far-infrared [O I] line strengths cannot be directly used as diagnostics of density and temperature.