Far-IR Observations of Gas and Dust in the Unusual 49 Ceti Disk

We present Herschel Space Observatory far-IR imaging and spectroscopy of 49 Cet, an unusual circumstellar disk around a nearby young A1V star. The system is famous for showing the dust properties of a debris disk, but the gas properties of a low-mass protoplanetary disk. Photometry was obtained at 70 and 160 microns; 49 Cet is significantly extended in the 70 micron image, spatially resolving the outer dust disk for the first time. Spectra covering small wavelength ranges centered on eight atomic, ionic, and molecular emission lines were obtained, including [OI] 63 microns and [CII] 158 microns. The CII line was detected at the 5-σ level; no other emission lines were seen. An estimate of the amount of circumstellar atomic gas implied by the CII emission is presented. A simple model of the new SED confirms the two-component structure of the disk: a warm inner disk and a cold outer disk that produces most of the observed excess. Finally, we briefly discuss preliminary thermochemical modeling of the 49 Cet gas and dust disk, and what the results suggest for the evolutionary status of the material in this important disk.