DETECTION OF THE DETACHED DUST SHELL OF U ANTLIAE AT MID-INFRARED WAVELENGTHS WITH AKARI/IRC

We report mid-infrared (MIR) imaging observations of the carbon star U Ant made with the Infrared Camera (IRC) on board AKARI. Subtraction of the artifacts and extended point-spread function of the central star reveals the detached dust shell around the carbon star at MIR wavelengths (15 and 24 mu m) for the first time. The observed radial brightness profiles of the MIR emission are well explained by two shells at 43 '' and 50 '' from the central star detected in optical scattered light observations. Combining Herschel/PACS, AKARI/FIS, and AKARI/IRC data, we obtain the infrared spectral energy distribution (SED) of the thermal emission from the detached shell of U Ant in a wide infrared spectral range of 15-160 mu m. Thermal emission of amorphous carbon grains with a single temperature cannot account for the observed SED from 15 to 160 mu m: it underestimates the emission at 15 mu m. Alternatively, the observed SED is fitted by the model in which amorphous carbon grains in the two shells have different temperatures of 60 and 104 K, which allocates most dust mass in the shell at 50 ''. This supports the previous suggestion that the 43 '' shell is gas-rich and the 50 '' shell is dust-rich. We suggest a possibility that the segregation of the gas and dust resulting from the drift motion of submicron-sized dust grains relative to the gas and that the hot dust component associated with the gas-rich shell is composed of very small grains that are strongly coupled with the gas.