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- Pasquini, L., et al.
(författare)
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Detailed chemical composition of the open cluster IC 4651: The iron peak, α elements, and Li
- 2004
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 1432-0746 .- 0004-6361. ; 424, s. 951-963
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Tidskriftsartikel (refereegranskat)abstract
- We present a detailed chemical analysis of 22 stars along thecolour-magnitude sequence of the intermediate-age (1.7 Gyr) open clusterIC 4651, based on high-resolution, high S/N ratio spectra from UVES/VLT.IC 4651 thus becomes one of the few open clusters for which a detailedcomposition analysis exists for stars spanning 3.5 mag, from solar-typemain-sequence stars to giants above the RGB clump. In a strictcomparison with the Sun, we find for the cluster a well-defined Feabundance of [Fe/H] = 0.10 ± 0.03 (internal errors), with areddening E(b-y) = 0.091. We also derive abundances for the αelements Mg, Si, Ti, and Ca and find a moderate enhancement of the threeformer elements, in excellent agreement with the results for field starsof similar Fe abundance. Among the Fe group elements, Cr and Ni areslightly overabundant ([X/Fe] ∼ 0.05). As also observed in M 67, theNa abundance among the giants is more than 0.2 dex higher than in thedwarfs. We interpret this effect as real, and due to dredge-up of23Na in the giants. Four turnoff stars, all fairly rapidrotators (v sin i ≥ 25 km s-1), appear to have higher[Fe/H] than the other stars; our tests show that, while a spuriousenhancement of 0.1 dex can be produced by the effect of high rotation onour spectral analysis, this cannot explain the whole difference. Thesestars appear overabundant because we tend to overestimate theireffective temperatures by forcing excitation equilibrium. Li abundanceshave been computed for all the stars and show a well-defined pattern:the Solar-type stars have an almost constant Li abundance, just belowthat of the Hyades, and the Li-dip is pronounced and well determined.Turnoff stars just above the dip have a ``cosmic'' Li abundance, butwithin a very small range of magnitudes (0.25 mag) higher on theturnoff, the Li level drops by more than a factor 10. This cannot be duejust to dilution; rather some extra mixing is required. Among thegiants, two probable clump stars show detectable Li, while all the other(likely) RGB stars do not - as is also observed in the similar clusterNGC 3680. None of these patterns can be explained by classical stellarevolution models. Again, some extra mixing is required. We show thatrotating stellar models including the most recent developments formeridional circulation and turbulence by shear instabilities explainvery well the behaviour of the lithium abundance along thecolour-magnitude diagram of IC 4651, including subgiant and giantphases. The possibility remains open that the giants exhibiting thehighest lithium abundances are actual RGB bump stars which have justbeen through the so-called ``lithium flash".Based on observations collected at the ESO VLT, Paranal Observatory,Chile.
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