Herschel Observations of Gas and Dust in the Unusual 49 Ceti Debris Disk

We present far-IR/sub-mm imaging and spectroscopy of 49 Ceti, 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. The data were acquired with the Herschel Space Observatory PACS and SPIRE
instruments, largely as part of the “Gas in Protoplanetary Systems” (GASPS) Open Time Key Programme. Disk
dust emission is detected in images at 70, 160, 250, 350, and 500 micron; 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 and molecular emission lines were obtained, including [O i] 63 micron and [C ii] 158 micron. The C ii line
was detected at the 5 sigma level—the first detection of atomic emission from the disk. No other emission lines were
seen, despite the fact that the Oi line is the brightest one observed in Herschel protoplanetary disk spectra. We
present an estimate of the amount of circumstellar atomic gas implied by the C ii emission. The new far-IR/sub-mm
data fills in a large gap in the previous spectral energy distribution (SED) of 49 Cet. A simple model of the new
SED confirms the two-component structure of the disk: warm inner dust and cold outer dust that produces most of
the observed excess. Finally, we discuss preliminary thermochemical modeling of the 49 Cet gas/dust disk and our
attempts to match several observational results simultaneously. Although we are not yet successful in doing so, our
investigations shed light on the evolutionary status of the 49 Cet gas, which might not be primordial gas but rather
secondary gas coming from comets.