Fuse Observations of Possible Infalling Planetesimals in the 51 Ophiuchi Circumstellar Disk

We present the first observations of the circumstellar (CS) disk system 51 Ophiuchi with the Far Ultraviolet Spectroscopic Explorer. We detect several absorption lines arising from the unusual metastable atomic species N I (D), N I (P ), and S II (D). These levels lie 1.8 – 3.6 eV above the ground level and have radiative decay lifetimes of 2 days or less, indicating that the lines arise from warm CS gas. The high S/N FUSE spectra, obtained six days apart, also show time-variable absorption features arising from N I, N II, O I (D), and Fe III, which are redshifted with respect to the stellar velocity. The resolved redshifted absorption extends over many tens of km s (40 for N I, 100 for N II, 65 for O I (D), and 84 for Fe III). We calculate column densities for all the variable infalling CS gasses, using the apparent optical depth method. The Fe III and N II infalling gasses must be produced through collisional ionization, and the ionization fraction of nitrogen suggests a gas temperature between 20000 and 34000 K. The infalling gas shows a peculiar, non-solar composition, with nitrogen and iron more abundant than carbon. We also set upper limits on the line-of-sight column densities of H2 and CO. These observations strengthen the connection between 51 Oph and the older debris-disk system β Pictoris, and indicate that there may be infalling planetesimals in the 51 Oph system. Subject headings: stars: individual (51 Ophiuchi) — circumstellar matter — planetary systems: formation — comets: general