Lithium in the context of Multiple Populations in Globular Clusters

TL;DR

This paper explores the unexpected presence of Lithium in second-generation globular cluster stars, examining models of star formation, dilution, and AGB star yields to explain observed abundance patterns.

Contribution

It analyzes Lithium abundance patterns in globular cluster stars, considering the role of AGB stars and initial gas compositions, offering insights into multiple population formation models.

Findings

Lithium is observed in 2G stars despite expectations of destruction.

AGB star yields may explain high Lithium in some extreme 2G stars.

Modeling depends on initial Lithium assumptions from Big Bang or observed values.

Abstract

Multiple Populations represent the standard for Globular Clusters (GC): a fraction (10-50%) of their stars have the same elemental abundances of halo stars of similar metallicity (first generation, or 1G), but the other stars (second generation, 2G) are characterised by patterns of light elements abundances which resemble those typical of gas processed by proton-capture reactions at high temperature. Consequently, we should naively expect that Lithium is destroyed in the 2G stars, but instead it is generally observed, at abundances only slightly depleted with respect to the 1G stars. After discussing the models for the formation of multiple populations, I examine the role of dilution with pristine gas and the possible role of the Asymptotic Giant Branch (AGB) scenario in accounting for the Lithium patterns in GC stars. Super-AGB and AGB yields of Lithium, produced by the Cameron Fowler mechanism in the `Hot Bottom Burning' convective envelopes, may help to explain the peculiar high Lithium in a few extreme 2G stars. On the other hand, modeling the abundances in mild 2G stars depends explicitly on whether the initial Li in the gas forming the 1G stars and the diluting gas of the 2G ones is that predicted by the Big Bang nucleosynthesis or the ~3 times smaller value observed at the surface of halo dwarfs.