Lithium in the lower red giant branch of 5 Galactic globular clusters

Abstract

Lithium is one of the few elements produced during the Big Bang Nucleosynthesis in the early universe. Moreover, its fragility makes it useful as a proxy for stellar environmental conditions. As such, the lithium abundance in old systems is at the core of different astrophysical problems. Stars in the lower red giant branch allow studying globular clusters where main sequence stars are too faint to be observed. We use these stars to analyze the initial Li content of the clusters and compare it to cosmological predictions, to measure spreads in Li between different stellar populations, and to study signs of extra depletion in these giants. We use GIRAFFE spectra to measure the lithium and sodium abundances of lower red giant branch stars in 5 globular clusters. These cover an extensive range in metallicity, from [Fe/H]$\sim -0.7$ to [Fe/H]$\sim -2.3$ dex. We find that the lithium abundance in these lower red giant branch stars forms a plateau, with values from $\mathrm{A}(\mathrm{Li}{)}_{\mathrm{NLTE}}=0.84$ to $1.03$ dex, showing no clear correlation with metallicity. When using stellar evolutionary models to calculate the primordial abundance of these clusters, we recover values $\mathrm{A}(\mathrm{Li}{)}_{\mathrm{NLTE}}=2.1-2.3$ dex, consistent with the constant value observed in warm metal-poor halo stars, the Spite plateau. Additionally, we find no difference in the lithium abundance of first and second population stars in each cluster. We also report the discovery of a Li-rich giant in the cluster NGC3201, with $\mathrm{A}(\mathrm{Li}{)}_{\mathrm{NLTE}}=1.63\pm 0.18$ dex, where the enrichment mechanism is probably pollution from external sources.

Accepted for publication in A&A

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