Discovery of multiple dust shells beyond 1 arcmin in the circumstellar envelope of IRC +10216 using Herschel/PACS ^⋆

L. Decin^1,2, P. Royer¹, N. L. J. Cox¹, B. Vandenbussche¹, R. Ottensamer³, J. A. D. L. Blommaert¹, M. A. T. Groenewegen⁴, M. J. Barlow⁵, T. Lim⁶, F. Kerschbaum³, T. Posch³ and C. Waelkens¹

¹ Instituut voor Sterrenkunde, Katholieke Universiteit Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
e-mail: Leen.Decin@ster.kuleuven.be
² Sterrenkundig Instituut Anton Pannekoek, University of Amsterdam, Science Park 904, 1098 Amsterdam, The Netherlands
³ University of Vienna, Department of Astronomy, Türkenschanzstraße 17, 1180 Vienna, Austria
⁴ Royal Observatory of Belgium, Ringlaan 3, 1180 Brussels, Belgium
⁵ Dept. of Physics & Astronomy, University College London, Gower St, London WC1E 6BT, UK
⁶ Space Science and Technology Department, Rutherford Appleton Laboratory, Oxfordshire, OX11 0QX, UK

Received: 27 May 2011
Accepted: 24 August 2011

Abstract

We present new Herschel/PACS images at 70, 100, and 160 μm of the well-known, nearby, carbon-rich asymptotic giant branch star IRC+10216 revealing multiple dust shells in its circumstellar envelope. For the first time, dust shells (or arcs) are detected until 320′′. The almost spherical shells are non-concentric and have an angular extent between  ~40° and  ~200°. The shells have a typical width of 5′′–8′′, and the shell separation varies in the range of  ~10′′–35′′, corresponding to  ~500 –1 700 yr. Local density variations within one arc are visible. The shell/intershell density contrast is typically  ~4, and the arcs contain some 50% more dust mass than the smooth envelope. The observed (nested) arcs record the mass-loss history over the past 16 000 yr, but Rayleigh-Taylor and Kelvin-Helmholtz instabilities in the turbulent astropause and astrosheath will erase any signature of the mass-loss history for at least the first 200 000 yr of mass loss. Accounting for the bowshock structure, the envelope mass around IRC+10216 contains  >2 M_⊙ of gas and dust mass. It is argued that the origin of the shells is related to non-isotropic mass-loss events and clumpy dust formation.