EDP Sciences
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Volume 365, Number 3, January III 2001
Page(s) 465 - 475
Section Formation, structure and evolution of stars
DOI http://dx.doi.org/10.1051/0004-6361:20000146

A&A 365, 465-475 (2001)
DOI: 10.1051/0004-6361:20000146

HD 172481: A super lithium-rich metal-deficient post-AGB binary with a red AGB companion

M. Reyniers and H. Van Winckel

Instituut voor Sterrenkunde, K.U. Leuven, Celestijnenlaan 200B, 3000 Leuven, Belgium

(Received 29 June 2000 / Accepted 16 October 2000 )

We present in this paper a study on the peculiar supergiant HD 172481. Its spectral type (F2Ia), high galactic latitude (b = -10 fraction of degree37), circumstellar dust, high radial velocity and moderate metal deficiency ([Fe/H] = -0.55) confirm the post-AGB character of this object. A detailed chemical analysis shows slight but real s-process overabundances, however no CNO-enhancement was detected. Furthermore, the spectral energy distribution and the TiO bands in the red part of the spectrum reveal a red luminous companion. The luminosity ratio of the hot F type component and this cool M type companion $L_{\rm F}/L_{\rm M}$ is derived for a reddening of E(B-V)=0.44 ( $L_{\rm F}/L_{\rm M} \sim 1.8$) and indicates that the companion must also be strongly evolved and probably evolving along the AGB. Neither our photometric data-set, nor our radial velocity monitoring show evidence for orbital variability which may indicate that the period is too large for direct binary interaction. Most interestingly, a strong lithium resonance line is detected, which yields an abundance of $\log\epsilon({\rm Li})$ = 3.6. Several explanations for this large lithium content are explored.

Key words: stars: abundances -- binaries: spectroscopic -- stars: chemically peculiar -- stars: individual: HD 172481 -- stars: AGB and post-AGB -- stars: evolution

Offprint request: M. Reyniers, maarten.reyniers@ster.kuleuven.ac.be

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