Surprising detection of an equatorial dust lane on the AGB star IRC+10216^⋆,⋆⋆

¹ Institut für Astrophysik, Georg-August-Universität Göttingen, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany
e-mail: Jeffers@astro.physik.uni-goettingen.de
² Leiden Observatory, PO Box 9513, 2300 RA Leiden, The Netherlands
³ Sterrenkundig Instituut Anton Pannekoek, Universiteit van Amsterdam, Postbus 94249, 1090 GE Amsterdam, The Netherlands
⁴ SRON, Netherlands Institute for Space Research, 3584 CA Utrecht, The Netherlands
⁵ Departamento de Fisica y Astronomia, Universidad de Valparaiso, Valpariso, Chile
⁶ Dept. of Astrophysics/IMAPP, Radbound Universiteit Nijmegen, Heijendaalseweg 135, 6525 AJ Nijmegen, The Netherlands
⁷ Astrophysics Research Institute, Liverpool John Moores University, 146 Brownlow Hill, Liverpool L3 5RF, UK
⁸ Instituut voor Sterrenkunde, Celestijnenlaan 200 D, 3001 Heverlee (Leuven), Belgium

Received: 19 January 2014
Accepted: 3 July 2014

Abstract

Aims. Understanding the formation of planetary nebulae remains elusive because in the preceding asymptotic giant branch (AGB) phase these stars are heavily enshrouded in an optically thick dusty envelope.

Methods. To further understand the morphology of the circumstellar environments of AGB stars we observe the closest carbon-rich AGB star IRC+10216 in scattered light.

Results. When imaged in scattered light at optical wavelengths, IRC+10216 surprisingly shows a narrow equatorial density enhancement, in contrast to the large-scale spherical rings that have been imaged much further out. We use radiative transfer models to interpret this structure in terms of two models: firstly, an equatorial density enhancement, commonly observed in the more evolved post-AGB stars, and secondly, in terms of a dust rings model, where a local enhancement of mass-loss creates a spiral ring as the star rotates.

Conclusions. We conclude that both models can be used to reproduce the dark lane in the scattered light images, which is caused by an equatorially density enhancement formed by dense dust rather than a bipolar outflow as previously thought. We are unable to place constraints on the formation of the equatorial density enhancement by a binary system.