Carbon but No CO? Circumstellar Gas Absorption in FUV Spectra of the Unusual 49 Ceti Debris Disk

We present the first high-resolution HST-STIS FUV spectra of the nearby A1V star 49 Ceti. The system is famous for harboring a circumstellar disk that shows the dust properties of a debris disk, but the gas properties of a low-mass protoplanetary disk. It is one of only two or three bona fide debris disks that show abundant sub-mm CO emission. 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. The spectra show many narrow circumstellar gas lines arising from several atomic species, including neutral carbon, a gas not seen in the diffuse interstellar medium. We present an accurate measurement of the line of sight column density, obtained from an unsaturated, spin-forbidden line. But carbon monoxide, the gas we expected to be most abundant, is not seen. We discuss possible explanations for this surprising observation, and the next steps in analyzing this rich dataset.