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Astron. Astrophys. 351, 273-282 (1999)

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The formation of lithium lines in the atmospheres of super Li-rich AGB stars

C. Abia 1, Y. Pavlenko 2 and P. de Laverny 3

1 Dpt. Física Teórica y del Cosmos, Universidad de Granada, 18071 Granada, Spain (cabia@goliat.ugr.es)
2 The Main Astronomical Observatory of NAS, Golosiiv woods, 252650 Kyiv-22, Ukraine (yp@mao.kiev.ua)
3 Observatoire de la Cote d'Azur, Dpt. Fresnel UMR 6528, B.P. 4229, 06304 Nice Cedex 04, France (laverny@obs-nice.fr)

Received 22 February 1999 / Accepted 25 May 1999

Abstract

The formation of lithium lines in the atmosphere of C-rich giants is discussed. LTE and NLTE approximations are used to model lithium lines in the spectra of super Li-rich AGB stars. The system of equations of the statistical balance of neutral Li in plane-parallel model atmospheres is solved for a 20-level atom model. JOLA and line-by-line models of molecular absorption are used to compute synthetic spectra as well as the opacity in the frequencies of bound-bound and bound-free transitions of the lithium lines. Curves of growth and synthetic spectra are computed in LTE and NLTE for several model atmospheres of different [FORMULA] and C/O ratios for four lithium absorptions, namely: the [FORMULA]4603, [FORMULA]6104, [FORMULA]6708 and [FORMULA]8126 Å Li I lines. The sensitivity of NLTE effects on [FORMULA] and the C/O ratio is discussed. We found that NLTE mainly affects the resonance line doublet ([FORMULA]) while the impact of NLTE effects on the lithium subordinate lines, formed in the inner regions of C-rich giant atmospheres, is rather weak. Therefore the use of these lines is recommended for Li determination in AGB stars. However, in no case can we properly account for the formation of Li lines in AGB stars until sphericity, velocity stratifications, dust, chromospheres and other related phenomena, which are in fact present in AGB star atmospheres, are considered. Our results are used to derive Li abundances in three super-Li rich C-stars taking into account NLTE effects. Finally, the net Li yield from this class of stars into the interstellar medium is reconsidered.

Key words: stars: abundances – stars: AGB and post-AGB – stars: carbon

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© European Southern Observatory (ESO) 1999

Online publication: November 2, 1999
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