Astron. Astrophys. 323, 449-460 (1997)

A double dust shell surrounding the carbon star U Antliae

H. Izumiura ^{* 1}, L.B.F.M. Waters ^{2, 1}, T. de Jong ^{1, 2}, C. Loup ³, Tj.R. Bontekoe ⁴ and D.J.M. Kester <sup>1

1</sup> SRON Laboratory for Space Research Groningen, P.O.Box 800, 9700 AV Groningen, The Netherlands
² Astronomical Institute Anton Pannekoek, University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
³ Institut d'Astrophysique de Paris, CNRS, 98 bis bd Arago, F-75014 Paris, France
⁴ Bontekoe Data Consultancy, Herengracht 47, 2312 LC Leiden, The Netherlands

Received 12 February 1996 / Accepted 5 August 1996

Abstract

We have investigated the N-type carbon star U Ant in high resolution IRAS images. We find that the star shows two extended dust shell components and that these two shells are also clearly present in the original survey scan data.

We have fitted a double dust shell model with spherical symmetry to the data to obtain the physical quantities of the shells. The inner dust shell component is related directly to the detached gas envelope detected in the mm-wave CO lines in previous studies, while the outer shell has an inner radius of about 3' but has no CO counterpart. The projected separation of the inner edges of the two shells, which is insensitive to any of the model parameters, is derived to be 141"-148". Our results together with the CO observations show that the mass loss rate varies by two orders of magnitude along the AGB evolution.

We are able to deduce the distance, interpulse period, core mass, and luminosity of the star selfconsistently, assuming that the two detached shells are related directly to two consecutive thermal pulses along the AGB evolution of this star. They are 324pc, 1.0 10⁴ years, , and 9.4 for the first thermal pulse stage, and 436pc, 1.4 10⁴ years, 0.77 , and 1.7 for the full amplitude thermal pulse stage, assuming an expansion velocity of 21 km s^-1 for both shells. The implied progenitor mass of U Ant is 3-5 . This method can be applied to other AGB stars with a multiple dust shell to be detected in future observations, which provides a way to determine reliable physical quantities of AGB stars.

Key words: stars: carbon stars: distances stars: evolution stars: individual: U Ant stars: mass-loss stars: AGB and post-AGB

* On leave Okayama Astrophysical Observatory, Kamogata, Asakuchi, Okayama 719-02, Japan

Send offprint requests to: H. Izumiura, Okayama Astrophysical Observatory, Kamogata, Asakuchi, Okayama 719-02, Japan

© European Southern Observatory (ESO) 1997

Online publication: June 5, 1998

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