Revisiting the ultraviolet spectroscopy of the eta Tel edge-on debris disk

We revisit the ultraviolet absorption spectroscopy of the edge-on debris disk surrounding the A0V star eta Telescopii. Previous work found absorption components at four velocities (about-23, -18, -10, -1 km s-1), with the most blueshifted component (-23 km s-1) interpreted as a likely disk wind. However, optical spectroscopy of eta Tel and other nearby stars in projection demonstrate that the-23 km s-1 component is likely interstellar in origin. We find that there are three interstellar components toward this sight line (-23, -18, -10 km s-1), but that the fourth component near-1 km s-1, which was only detected in O I, is inconsistent with an interstellar origin and could be circumstellar. We place a 3- sigma upper limit on the C/O ratio of the-1 km s-1 gas (log C/O < -2.1), finding that it is consistent with Earth and solar system comet abundances. However, the abundance is inconsistent with the carbon-rich disks of beta Pic (A5V) and 49 Cet (A1V), probably because eta Tel (A0V) is a warmer star imposing greater levels of radiation pressure on carbon atoms in the disk. A low C/O ratio is also inconsistent with Herschel's [CII] detection toward eta Tel and may indicate that carbon gas is misaligned from the line of sight or variable in time.