How stars in globular clusters reveal the depletion of the Spite plateau of lithium

TL;DR

This paper reviews 15 years of research on globular cluster stars, revealing systematic abundance trends and supporting models of atomic diffusion with mixing to explain lithium depletion and the cosmological lithium problem.

Contribution

It provides observational evidence of heavy-element abundance trends and supports atomic diffusion models with mixing as a solution to lithium depletion.

Findings

Observed heavy-element abundance trends from ~0.1 to 0.3 dex.

Turn-off and subgiant stars show lower abundances than giants.

Atomic diffusion models with mixing explain abundance trends and lithium depletion.

Abstract

I summarize the results of 15 years of research on the surface abundances of stars in globular clusters. Globular-cluster stars afford a unique view of the physical processes that shape the surface abundances of stars on and beyond the Spite plateau, as one can study stars in different evolutionary phases with the same composition at birth. The main result is the finding of observational trends in surface abundances of heavy elements (Mg, Ca, Ti, Cr, Fe, Ba) that range from ~0.1 dex (M4 at [Fe/H]=-1.1) to ~0.3 dex (M30 at [Fe/H]=-2.3) with turn-off and subgiant stars generally showing lower abundances than giant stars. Any model that tries to resolve the cosmological lithium discrepancy by means of stellar physics must simultaneously describe these systematic heavy-element trends between dwarfs and giants. Models of atomic diffusion moderated by some additional mixing can indeed explain the observed abundance trends and make quantitative predictions for lithium depletion. The inferred birth-cloud lithium abundances of Spite-plateau stars are higher by ~0.3 dex leaving a small (~0.15 dex) nominal offset to the primordial value.