Based on observations collected at the European Southern Observatory, Paranal, Chile (Proposal Number 68.B-0320).

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Tables 1 and 2 and the appendices are only available in electronic form at http://www.edpsciences.org

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... ${\rm [Fe/H]} \leq -1.5$

${\rm [A/B]} = \log ( N_{\rm A} / N_{\rm B} )_\star - \log (N_{\rm A} / N_{\rm B})_{\rm\hbox{$\odot$ }}$ where $N_{\rm X}$ are number densities.

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... database

http://www.astro.univie.ac.at/~vald/

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... Database

http://physics.nist.gov/cgi-bin/AtData/main_asd

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... $\log \epsilon_{\rm Fe}$

We define the abundance parameter for element X in the standard notation $\log \epsilon_{\rm X} = \log(N_{\rm X}/N_{\rm H}) + 12$ where $N_{\rm X}$ are number densities.

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... ${\rm [Ba/Eu]}>0.5$

As described in Appendix A, we have assumed r-process isotopic composition of Ba in all stars which may lead to underestimation of the Ba abundance and thus Ba/Eu in stars with significant s-process contributions. Similarly, we assume no ¹³C which may lead to errors in the C abundances for such stars.

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