Far ultraviolet spectroscopic explorer 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 (FUSE). We detect several absorption lines arising from the unusual metastable atomic species N I (²D), N I (²P), and S II (²D). 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 6 days apart, also show time-variable absorption features arising from N I, N II, O I (¹D), and Fe III, which are redshifted with respect to the stellar velocity. The resolved redshifted absorption extends over many tens of km s^-1 (40 for N I, 100 for N II, 65 for O I (¹D), 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 20,000 and 34,000 K. The infalling gas shows a peculiar, nonsolar composition, with nitrogen and iron more abundant than carbon. We also set upper limits on the line-of-sight column densities of H₂and 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.