Atomic diffusion and mixing in old stars IV: Weak abundance trends in the globular cluster NGC 6752

TL;DR

This study investigates atomic diffusion effects in stars of the globular cluster NGC 6752, revealing weak abundance trends that support models including atomic diffusion and mixing, and helping reconcile lithium observations with Big Bang predictions.

Contribution

It provides observational evidence for atomic diffusion with mixing in globular cluster stars and refines primordial lithium abundance estimates.

Findings

Weak abundance trends observed for Fe, Sc, Ti, Ca

Model including atomic diffusion and mixing explains trends

Reconciles stellar lithium with Big Bang nucleosynthesis predictions

Abstract

Atomic diffusion in stars can create systematic trends of surface abundances with evolutionary stage. Globular clusters offer useful laboratories to put observational constraints on this theory as one needs to compare abundances in unevolved and evolved stars, all drawn from the same stellar population. In this paper, we show the results of an abundance study of stars in the globular cluster NGC6752 which shows weak but systematic abundances trends with evolutionary phase for Fe, Sc, Ti and Ca. The trends are best explained by a stellar structure model including atomic diffusion with efficient additional mixing. The model allows to correct for sub-primordial stellar lithium abundances of the stars on the Spite plateau, and to match it to the WMAP-calibrated Big-Bang nucleosynthesis predictions to within the mutual 1-sigma errors.