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

¹ Okayama Astrophysical Observatory (OAO), National Astronomical Observatory of Japan (NAOJ), 3037-5 Honjo, Kamogata, Asakuchi, Okayama 719-0232, Japan
e-mail: izumiura@oao.nao.ac.jp
² Department of Physics and Astronomy, University of Denver, 2112 E. Wesley Ave., Denver, CO 80208, USA
e-mail: tueta@du.edu
³ Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, 3-1-1 Yoshinodai, Chuo-ku, Sagamihara, Kanagawa 252-5210, Japan
e-mail: yamamura@ir.isas.jaxa.jp
⁴ Kiso Observatory, Institute of Astronomy, Graduate School of Science, The University of Tokyo, 10762-30 Mitake, Kiso, Nagano 397-0101, Japan
e-mail: matsunaga@kiso.ioa.s.u-tokyo.ac.jp; mito@kiso.ioa.s.u-tokyo.ac.jp
⁵ Astronomical Institute, Graduate School of Science, Tohoku University, 6-3 Aramaki Aoba, Aoba-ku, Sendai, Miyagi 980-8578, Japan
e-mail: yita@astr.tohoku.ac.jp
⁶ UCL-Institute of Origins, Astrophysics Group, Department of Physics and Astronomy, University of College London, Gower Street, London WC1E 6BT, UK
e-mail: mikako@star.ucl.ac.uk
⁷ UCL-Institute of Origins, Mullard Space Science Laboratory, University of College London, Holmbury St. Mary, Dorking, Surrey RH5 6NT, UK
⁸ Institute of Astronomy, Graduate School of Science, The University of Tokyo, 2-21-1 Osawa, Mitaka, Tokyo 181-0015, Japan
e-mail: nakada@kiso.ioa.s.u-tokyo.ac.jp; hina@ioa.s.u-tokyo.ac.jp; ttanabe@ioa.s.u-tokyo.ac.jp
⁹ Gunma Astronomical Observatory, 6860-86 Nakayama, Takayama, Agatsuma, Gunma 377-0702, Japan
e-mail: osamu@astron.pref.gunma.jp

Received: 28 May 2010
Accepted: 12 November 2010

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 ~10⁴ 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 R_in of 101–107″[(2.5–2.6)  ×  10¹⁷ cm], thickness that covers a half of the total dust mass ΔR_hm of 16–23′′[(3.8–5.6)  ×  10¹⁶ cm], 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)  ×  10^-4   (κ₁₀₀/25)^-1 M_⊙, where κ₁₀₀ is the dust absorptivity at 100 μm in units of cm² g^-1. The dust mass-loss rate at R_in is found to be (1.8–9.6)  ×  10^-8(κ₁₀₀/25)^-1(v_e/15)   M_⊙   yr^-1, 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^-1. 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.

Conclusions. 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.