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Lithium abundances of main-sequence and subgiant stars in the globular cluster NGC 6397

Published online by Cambridge University Press:  18 January 2010

J. I. González Hernández ,
P. Bonifacio ,
E. Caffau ,
M. Steffen ,
H.-G. Ludwig ,
N. Behara ,
L. Sbordone ,
R. Cayrel  and
S. Zaggia
J. I. González Hernández
Affiliation:
GEPI, Observatoire de Paris, CNRS, Université Paris Diderot, Place Jules Janssen, 92190 Meudon, France email: [email protected] Cosmological Impact of the First Stars (CIFIST) Marie Curie Excellence Team
P. Bonifacio
Affiliation:
GEPI, Observatoire de Paris, CNRS, Université Paris Diderot, Place Jules Janssen, 92190 Meudon, France email: [email protected] Cosmological Impact of the First Stars (CIFIST) Marie Curie Excellence Team Istituto Nazionale di Astrofisica, Observatorio Astronomico di Trieste, Italy
E. Caffau
Affiliation:
GEPI, Observatoire de Paris, CNRS, Université Paris Diderot, Place Jules Janssen, 92190 Meudon, France email: [email protected]
M. Steffen
Affiliation:
Astrophysikalisches Institut Potsdam, An der Sternwarte 16, D-14482 Potsdam, Germany
H.-G. Ludwig
Affiliation:
GEPI, Observatoire de Paris, CNRS, Université Paris Diderot, Place Jules Janssen, 92190 Meudon, France email: [email protected] Cosmological Impact of the First Stars (CIFIST) Marie Curie Excellence Team
N. Behara
Affiliation:
GEPI, Observatoire de Paris, CNRS, Université Paris Diderot, Place Jules Janssen, 92190 Meudon, France email: [email protected] Cosmological Impact of the First Stars (CIFIST) Marie Curie Excellence Team
L. Sbordone
Affiliation:
GEPI, Observatoire de Paris, CNRS, Université Paris Diderot, Place Jules Janssen, 92190 Meudon, France email: [email protected] Cosmological Impact of the First Stars (CIFIST) Marie Curie Excellence Team
R. Cayrel
Affiliation:
GEPI, Observatoire de Paris, CNRS, Université Paris Diderot, Place Jules Janssen, 92190 Meudon, France email: [email protected]
S. Zaggia
Affiliation:
INAF, Osservatorio Astronomico di Padova, Vicolo dell'Osservatorio 5, Padua 35122, Italy
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Abstract

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We present FLAMES/GIRAFFE spectroscopy obtained with the Very Large Telescope (VLT). Using these observations, we have been able (for the first time) to observe the Lii doublet in the main-sequence (MS) stars of a globular cluster. We also observed Li in a sample of subgiant (SG) stars of the same B − V colour. Our final sample is composed of 84 SG and 79 MS stars. In spite of the fact that SG and MS stars span the same temperature range, we find that the equivalent widths of the Lii doublet in SG stars are systematically greater than in MS stars, suggesting a higher Li content among SG stars. This is confirmed by our quantitative analysis, which makes use of both 1D and 3D model atmospheres. We find that SG stars show, on average, a higher Li abundance, by 0.1 dex, than MS stars. We also detect a positive slope of Li abundance with effective temperature: the higher the temperature the higher the Li abundance, both for SG and MS stars, although the slope is slightly steeper for MS stars. These results provide unambiguous evidence that the Li abundance changes with evolutionary state. The physical mechanisms that contribute to this are not yet clear, since none of the proposed models seem to describe accurately the observations. Whether such a mechanism can explain the cosmological lithium problem is still an open question.

Keywords

stars: abundancesstars: atmospheresstars: fundamental parametersstars: Population IIglobular clusters: individual (NGC 6397)
Type
Contributed Papers
Information
Proceedings of the International Astronomical Union , Volume 5 , Symposium S266: Star clusters: basic galactic building blocks throughout time and space , August 2009 , pp. 407 - 410
Copyright
Copyright © International Astronomical Union 2010

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