Detection of the detached dust shell of U Antliae at mid-infrared wavelengths with AKARI/IRC

TL;DR

This study uses AKARI/IRC mid-infrared imaging to detect and analyze the detached dust shell of the carbon star U Antliae, revealing two shells with different dust and gas compositions and temperatures.

Contribution

First mid-infrared imaging detection of U Antliae's detached dust shell, combined with spectral energy distribution modeling to reveal dust and gas segregation.

Findings

Detected two shells at 43" and 50" from the star.

Modeled SED indicates different temperatures for the shells (60 K and 104 K).

Supports the idea that the 43" shell is gas-rich and the 50" shell is dust-rich.

Abstract

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 PSF 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 that 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 previous suggestion that the 43" shell is gas-rich and the 50" one 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.