TheFUSESpectrum of the Planetary Nebula SwSt 1: Evidence for Inhomogeneities in the Gas and Dust

[Abridged] We present Far Ultraviolet Spectroscopic Explorer (FUSE)observations of the young, compact planetary nebula (PN) SwSt 1 along the lineof sight to its central star HD 167362. We detect circumstellar absorptionlines from several species against the continuum of the central star. Thephysical parameters of the nebula derived from the FUSE data differsignificantly from those found from emission lines. We derive an electrondensity n_e = 8800^{+4800}_{-2400} cm^{-3} from the column density ratio of theexcited S III fine structure levels, which is at least a factor of 3 lower thanall prior estimates. The gaseous iron abundance derived from the UV lines isquite high ([Fe/S] = -0.35+/-0.12), which implies that iron is notsignificantly depleted into dust. In contrast, optical and near-infraredemission 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 H_2absorption, to a limit N(H_2) < 7\times10^14 cm^{-2}, at least four orders ofmagnitude lower than the column density estimated from infrared H_2 emissionlines. Taken together, the lack of H_2 absorption, low n_e, and high gaseous Feabundance derived from the FUSE spectrum provide strong evidence that densestructures (which can shield molecules and dust from the destructive effects ofenergetic stellar photons) are not present along the line of sight to thecentral star. On the other hand, there is substantial evidence for dust,molecular material, and dense gas elsewhere in SwSt 1. Therefore, we concludethat the nebula must have an inhomogeneous structure.Comment: 46 pages, 4 figures. Accepted for publication in the Astrophysical Journa