A Radiatively Driven Wind from the eta Tel Debris Disk

We present far- and near-ultraviolet absorption spectroscopy of the similar to 23 Myr edge-on debris disk surrounding the A0V star eta Telescopii, obtained with the Hubble Space Telescope Space Telescope Imaging Spectrograph. We detect absorption lines from C i, C ii, O i, Mg ii, Al ii, Si ii, S ii, Mn ii, Fe ii, and marginally N i. The lines show two clear absorption components at -22.7 +/- 0.5 km s(-1) and -17.8 +/- 0.7 km s(-1), which we attribute to circumstellar (CS) and interstellar gas, respectively. CO absorption is not detected, and we find no evidence for star-grazing exocomets. The CS absorption components are blueshifted by -16.9 +/- 2.6 km s(-1) in the star's reference frame, indicating that they are outflowing in a radiatively driven disk wind. We find that the C/Fe ratio in the eta Tel CS gas is significantly higher than the solar ratio, as is the case in the beta Pic and 49 Cet debris disks. Unlike those disks, however, the measured C/O ratio in the eta Tel CS gas is consistent with the solar value. Our analysis shows that because eta Tel is an earlier type star than beta Pic and 49 Cet, with more substantial radiation pressure at the dominant C ii transitions, this species cannot bind the CS gas disk to the star as it does for beta Pic and 49 Cet, resulting in the disk wind.