Absorption-Line Spectroscopy of Planetary Nebulae with FUSE: Probing the Molecular, Atomic, and Ionized Gas

The central stars of planetary nebulae (PNe) are natural targets for FUSE due to their UV brightness. The FUSE spectra of many PNe show absorption features due to circumstellar material in species ranging from H_2 and neutrals in the photodissociation region (PDR) to ions resident in the H II region. We report results from a program designed to search for nebular components in the H_2 Lyman and Werner resonance lines that are responsible for the fluorescent excitation of H_2 in strong FUV radiation fields. Our failure to detect H_2 in absorption in several PNe with strong near-infrared H_2 emission indicates that the molecular material has an asymmetrical or clumpy distribution. We also detect enrichments in the s-process product Ge, find that Fe is not depleted into dust along at least one line of sight through a PN, and show that starlight fluorescence can affect the populations of the excited fine-structure levels of O I.