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Astron. Astrophys. 354, 135-149 (2000)

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A homogeneous study of the s-process in the 21 µm carbon-rich post-AGB objects *

Hans van Winckel  ** and Maarten Reyniers ***

Instituut voor Sterrenkunde, K.U.Leuven, Celestijnenlaan 200B, 3001 Leuven, Belgium

Received 2 July 1999 / Accepted 21 September 1999


We present in this paper a homogeneous photospheric abundance study, on the basis of the analysis of high resolution optical spectra, of six post-AGB objects displaying a 21µm circumstellar dust feature in their IR spectrum. The F-G spectral type of the 21µm stars make that a large range of elements including a wide variety of s-process elements, can be studied by their atomic lines. The high C/O-ratios together with the large overabundance of s-process elements prove that the objects are descendants of field carbon stars. We discuss in detail the s-process abundance distribution displayed by these 21µm stars and conclude that the 3rd dredge-up efficiency is closely related to the strength of the integrated neutron irradiation. The expected anti-correlation of the neutron irradiation with metallicity, on the other hand, contains a large intrinsic scatter. Finally we compare our results with other intrinsic and extrinsic s-process enriched objects and conclude that the post-AGB stars offer very useful complementary data to constrain the evolutionary models of AGB nucleosynthesis and dredge-up processes.

Key words: stars: evolution – stars: carbon – stars: abundances

* based on observations collected at the European Southern Observatory in Chile (61.E-0426), and at Roque de los Muchachos at La Palma Spain
** Postdoctoral fellow of the Fund for Scientific Research, Flanders
*** Scientific researcher of the Fund for Scientific Research, Flanders

Send offprint requests to: Hans Van Winckel

© European Southern Observatory (ESO) 2000

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