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Main-sequence and sub-giant stars in the globular cluster NGC 6397: The complex evolution of the lithium abundance

Published online by Cambridge University Press:  23 April 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: Jonay.Gonzalez-Hernandez@obspm.fr 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: Jonay.Gonzalez-Hernandez@obspm.fr 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: Jonay.Gonzalez-Hernandez@obspm.fr
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: Jonay.Gonzalez-Hernandez@obspm.fr 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: Jonay.Gonzalez-Hernandez@obspm.fr 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: Jonay.Gonzalez-Hernandez@obspm.fr 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: Jonay.Gonzalez-Hernandez@obspm.fr
S. Zaggia
Affiliation:
INAF - Osservatorio Astronomico di Padova, Vicolo dell'Osservatorio 5, Padua 35122, Italy
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Abstract

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Thanks to the high multiplex and efficiency of Giraffe at the VLT we have been able for the first time to observe the Li I doublet in the Main Sequence stars of a globular cluster. At the same time we observed Li in a sample of Sub-Giant stars of the same B-V colour.

Our final sample is composed of 84 SG stars and 79 MS stars. In spite of the fact that SG and MS span the same temperature range we find that the equivalent widths of the Li I doublet in SG stars are systematically larger than those in MS stars, suggesting a higher Li content among SG stars. This is confirmed by our quantitative analysis carried out making use of 1D hydrostatic plane-parallel models and 3D hydrodynamical simulations of the stellar atmospheres.

We derived the effective temperatures of stars in our the sample from Hα fitting. Theoretical profiles were computed using 3D hydrodynamical simulations and 1D ATLAS models. Therefore, we are able to determined 1D and 3D-based effective temperatures. We then infer Li abundances taking into account non-local thermodynamical equilibrium effects when using both 1D and 3D models.

We find that SG stars have a mean Li abundance higher by 0.1 dex than MS stars. This result is obtained using both 1D and 3D models. 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 an unambiguous evidence that the Li abundance changes with evolutionary status.

The physical mechanisms responsible for this behaviour are not yet clear, and none of the existing models seems to describe accurately these observations. Based on these conclusions, we believe that the cosmological lithium problem still remains an open question.

Keywords

Stars: abundancesfundamental parametersPopulation II – Galaxy: globular clusters: individual: NGC 6397
Type
Contributed Papers
Information
Proceedings of the International Astronomical Union , Volume 5 , Symposium S268: Light Elements in the Universe , November 2009 , pp. 257 - 261
Copyright
Copyright © International Astronomical Union 2010

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