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Detailed chemical composition of the open cluster IC 4651: The iron peak, α elements, and Li

Pasquini, L.; Randich, S.; Zoccali, M.; Hill, V.; Charbonnel, C. and Nordström, Birgitta LU (2004) In Astronomy and Astrophysics 424. p.951-963
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... (More)
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 of<SUP>23</SUP>Na in the giants. Four turnoff stars, all fairly rapidrotators (v sin i &geq; 25 km s<SUP>-1</SUP>), 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. (Less)
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author
organization
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type
Contribution to journal
publication status
published
subject
keywords
stars: evolution, stars: abundances, Galaxy: open clusters and associations: individual: IC 4651, stars: rotation
in
Astronomy and Astrophysics
volume
424
pages
951 - 963
publisher
EDP Sciences
external identifiers
  • wos:000224480900024
  • scopus:4644370492
ISSN
1432-0746
DOI
10.1051/0004-6361:20040240
language
English
LU publication?
yes
id
207d931d-67bd-4d44-aee2-19cf29dd6ff2 (old id 134159)
date added to LUP
2007-07-13 14:51:19
date last changed
2017-09-17 07:01:03
@article{207d931d-67bd-4d44-aee2-19cf29dd6ff2,
  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 of&lt;SUP&gt;23&lt;/SUP&gt;Na in the giants. Four turnoff stars, all fairly rapidrotators (v sin i &amp;geq; 25 km s&lt;SUP&gt;-1&lt;/SUP&gt;), 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.},
  author       = {Pasquini, L. and Randich, S. and Zoccali, M. and Hill, V. and Charbonnel, C. and Nordström, Birgitta},
  issn         = {1432-0746},
  keyword      = {stars: evolution,stars: abundances,Galaxy: open clusters and associations: individual: IC 4651,stars: rotation},
  language     = {eng},
  pages        = {951--963},
  publisher    = {EDP Sciences},
  series       = {Astronomy and Astrophysics},
  title        = {Detailed chemical composition of the open cluster IC 4651: The iron peak, α elements, and Li},
  url          = {http://dx.doi.org/10.1051/0004-6361:20040240},
  volume       = {424},
  year         = {2004},
}