Extreme Carbon Overabundance in the 49 Ceti Circumstellar Gas

We present analysis of C and O absorption in high-resolution HST-STIS FUV spectra of the nearby A1V star 49 Ceti. This disk system is one of the few showing the dust properties of a debris disk, but harboring relatively abundant molecular gas more characteristic of a low-mass protoplanetary disk. Since the disk is nearly edge-on, the line-of-sight to the central star passes through the disk, permitting sensitive probes of the circumstellar gas with absorption spectroscopy.Our FUV spectra show many narrow circumstellar gas lines arising from several atomic species, including neutral carbon (a gas not seen in the local ISM) and neutral oxygen. We present an estimate of the total carbon column density in the line-of-sight gas and limits on the oxygen column density. Comparing the carbon abundance to a previous measurement of the line-of-sight iron abundance, we see that the carbon is extremely overabundant relative to the solar abundance. A similar overabundance is seen in the Beta Pic disk gas, where the carbon brakes other gases from being rapidly blown out by radiation pressure. The carbon in the 49 Cet gas may play a similar role.