Lithium abundance in lower red giant branch stars of Omega Centauri

TL;DR

This study analyzes lithium, sodium, aluminum, and iron abundances in 199 lower red giant branch stars of Omega Centauri, revealing complex chemical patterns and differences from typical globular clusters, indicating diverse stellar populations and formation histories.

Contribution

It provides the first detailed chemical abundance analysis of lower red giant branch stars in Omega Centauri, highlighting unique lithium depletion patterns and multiple stellar populations.

Findings

Li distribution peaks at ~1 dex with a tail towards lower values

Li-poor stars are present at all metallicities

Omega Centauri hosts multiple stellar populations with distinct chemical signatures

Abstract

We present Li, Na, Al and Fe abundances of 199 lower red giant branch stars members of the stellar system Omega Centauri, using high-resolution spectra acquired with FLAMES at the Very Large Telescope. The A(Li) distribution is peaked at A(Li) ~ 1 dex with a prominent tail toward lower values. The peak of the distribution well agrees with the lithium abundances measured in lower red giant branch stars in globular clusters and Galactic field stars. Stars with A(Li) ~ 1 dex are found at metallicities lower than [Fe/H] ~ -1.3 dex but they disappear at higher metallicities. On the other hand, Li-poor stars are found at all the metallicities. The most metal-poor stars exhibit a clear Li-Na anticorrelation, with about 30% of the sample with A(Li) lower than ~ 0.8 dex, while in normal globular clusters these stars represent a small fraction. Most of the stars with [Fe/H] > -1.6 dex are Li-poor and Na-rich. The Li depletion measured in these stars is not observed in globular clusters with similar metallicities and we demonstrate that it is not caused by the proposed helium enhancements and/or young ages. Hence, these stars formed from a gas already depleted in lithium. Finally, we note that Omega Centauri includes all the populations (Li-normal/Na-normal, Li-normal/Na-rich and Li-poor/Na-rich stars) observed, to a lesser extent, in mono-metallic GCs.