Extended dust shell of the carbon star U Hydrae observed with AKARI

Abstract

Context. Low- to intermediate-mass stars lose a significant fraction of their mass while they are on the asymptotic giant branch (AGB). This mass loss is considered to determine the final stages of their evolution. The material ejected from the stellar photosphere forms a circumstellar envelope in its surroundings. Layers of circumstellar envelope constitute the footprint of mass-loss history. Aims. Our aim is to probe the mass-loss history in the carbon star U Hya in the last ∼10 4 years by investigating the distribution of dust in the circumstellar envelope with high spatial resolution. Methods. We observed U Hya in the far-infrared (FIR) at 65, 90, 140, and 160 μm simultaneously, using the slow scan observing mode of the far-infrared surveyor (FIS) aboard the infrared astronomical satellite AKARI. It produced a map of ∼10 � × 40 � in size in each band. Results. The FIS maps reveal remarkably circular, ring-like emission structure almost centered on the star, showing the presence of a detached, spherical dust shell. A hollow dust shell model gives the inner radius Rin of 101–107 �� [(2.5–2.6) × 10 17 cm], thickness that covers a half of the total dust mass ΔRhm of 16–23 �� [(3.8–5.6) × 10 16 cm], which gives ΔRhm/Rin ∼ 0.2, and the power-law index of the dust opacity distribution of 1.10–1.15. The dust mass in the shell is well-constrained to be (0.9–1.4) × 10 −4 (κ100/25) −1 M� , where κ100 is the dust absorptivity at 100 μm in units of cm 2 g −1 . The dust mass-loss rate at Rin is found to be (1.8–