The Composition, Excitation, and Physical State of Atomic Gas in the Debris Disk Surrounding 51 Oph

We measured 304 absorption features in the ultraviolet and visible spectra of the star 51 Oph, which is known to have a debris disk with a high inclination. We analyzed the relative populations of atoms in excited fine-structure and metastable levels that are maintained by optical pumping and collisional excitation by electrons, and we found that most of the gas is situated at about 6AU from the star, has an electron volume density $10^5 < n(e) < 3\times 10^6{\rm cm}^{-3}$, and a temperature T=8000K. Our interpretations reveal that the gas is partly ionized, has a column density of neutral hydrogen equal to $10^{21}\,{\rm cm}^{-2}$, and has a composition similar to that of a mildly depleted interstellar medium or that of Jupiter-family comets. Compared to results for disks around some other stars, such as $\beta$ Pic and 49 Cet, we find surprisingly little neutral carbon. No molecular features were detected, which indicates that our line of sight misses the molecule-rich central plane of the disk. The tilt of the disk is also validated by our being able to detect resonant scattering of the starlight by oxygen atoms.