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Astron. Astrophys. 343, 507-518 (1999)


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The rapid evolution of the born-again giant Sakurai's object

M. Asplund 1, D.L. Lambert 2, T. Kipper 3, D. Pollacco 4 and M.D. Shetrone 5

1 NORDITA, Blegdamsvej 17, DK-2100 Copenhagen O, Denmark
2 Department of Astronomy, University of Texas, Austin, TX 78712, USA
3 Tartu Observatory, Tõravere, EE2444, Estonia
4 Isaac Newton Group, Apartado de Correos 368, Santa Cruz de La Palma, E-38780 Tenerife, Spain
5 McDonald Observatory, P.O. Box 1337, Fort Davis, TX 79734, USA

Received 7 September 1998 / Accepted 30 November 1998

Abstract

The extraordinarily rapid evolution of the born-again giant Sakurai's object following discovery in 1996 has been investigated. The evolution can be traced both in a continued cooling of the stellar surface and dramatic changes in chemical composition on a timescale of a mere few months. The abundance alterations are the results of the mixing and nuclear reactions which have ensued due to the final He-shell flash which occurred during the descent along the white dwarf cooling track. The observed changes in the H and Li abundances can be explained by ingestion and burning of the H-rich envelope and Li-production through the Cameron-Fowler mechanism. The rapidly increasing abundances of the light s-elements (including Sc) are consistent with current s-processing by neutrons released from the concomitantly produced 13C. However, the possibility that the s-elements have previously been synthesized during the AGB-phase and only mixed to the surface in connection with the final He-shell flash in the pre-white dwarf cannot be convincingly ruled out either. Since Sakurai's object shows substantial abundance similarities with the R CrB stars and has recently undergone R CrB-like visual fading events, the "birth" of an R CrB star may have been witnessed for the first time ever. Sakurai's object thus lends strong support for the suggestion that at least some of the R CrB stars have been formed through a final He-shell flash in a post-AGB star.

Key words: stars: variables: general – stars: individual: Sakurai's object – stars: AGB and post-AGB – stars: evolution – stars: abundances

Send offprint requests to: M. Asplund (martin@nordita.dk)

© European Southern Observatory (ESO) 1999

Online publication: March 1, 1999

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