2011A&A...528A..29I

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. Our aim is to probe the mass-loss history in the carbon star U Hya in the last ∼10⁴years by investigating the distribution of dust in the circumstellar envelope with high spatial resolution. We observed U Hya in the far-infrared (FIR) at 65, 90, 140, and 160µm 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'x40' in size in each band. 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 R_in of 101-107" [(2.5-2.6)x10¹⁷cm], thickness that covers a half of the total dust mass ΔR_hm of 16-23'' [(3.8-5.6)x10¹⁶cm], which gives ΔR_hm/R_in∼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)x10^–4(κ₁₀₀/25)^–1M_☉, where κ₁₀₀ is the dust absorptivity at 100 µm in units of cm2/g. The dust mass-loss rate at R_in is found to be (1.8-9.6)x10^–8(κ₁₀₀/25)^–1(v_e/15)M_☉/yr, which shows that the total mass-loss rate in the shell is at least an order of magnitude higher than the current rate, where v_e is the outflow velocity at R_in in units of km/s. An extension of FIR emission along PA~-70° is found out to ∼5' from the star, which is probably a ram-stripping wake by the ISM wind. We also find excess FIR emission that might indicate the earliest departure from spherical symmetry in the AGB mass loss inside the shell. The dust shell of U Hya is a hollow sphere and has an effective width that is narrower than the spatial resolution of the FIS. It could be formed as either a direct consequence of a thermal pulse, a result of two-wind interaction induced by a thermal pulse, a termination shock, or any combination of these processes.