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IRAS08281-4850 and IRAS14325-6428: two A-type post-AGB stars with
s-process enrichment | | M. Reyniers
; G.C. Van de Steene
; P.A.M. van Hoof
; H. Van Winckel
; | | Date: |
14 Jun 2007 | | Abstract: | One of the puzzling findings in the study of the chemical evolution of
(post-)AGB stars is why very similar stars (in terms of metallicity, spectral
type, infrared properties, etc...) show a very different photospheric
composition. We aim at extending the still limited sample of s-process enriched
post-AGB stars, in order to obtain a statistically large enough sample that
allows us to formulate conclusions concerning the 3rd dredge-up occurrence. We
selected two post-AGB stars on the basis of IR colours indicative of a past
history of heavy mass loss: IRAS08281-4850 and IRAS14325-6428. They are cool
sources in the locus of the Planetary Nebulae (PNe) in the IRAS colour-colour
diagram. Abundances of both objects were derived for the first time on the
basis of high-quality UVES and EMMI spectra, using a critically compiled line
list with accurate log(gf) values, together with the latest Kurucz model
atmospheres. Both objects have very similar spectroscopically defined effective
temperatures of 7750-8000K. They are strongly carbon and s-process enriched,
with a C/O ratio of 1.9 and 1.6, and an [ls/Fe] of +1.7 and +1.2, for IRAS08281
and IRAS14325 resp. Moreover, the spectral energy distributions (SEDs) point to
heavy mass-loss during the preceding AGB phase. IRAS08281 and IRAS14325 are
prototypical post-AGB objects in the sense that they show strong post 3rd
dredge-up chemical enrichments. The neutron irradiation has been extremely
efficient, despite the only mild sub-solar metallicity. This is not conform
with the recent chemical models. The existence of very similar post-AGB stars
without any enrichment emphasizes our poor knowledge of the details of the AGB
nucleosynthesis and dredge-up phenomena. We call for a very systematic chemical
study of all cool sources in the PN region of the IRAS colour-colour diagram. | | Source: | arXiv, arxiv.0706.2120 | | Services: | Forum | Review | PDF | Favorites | |
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