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TL;DR

This paper reviews evidence for atomic diffusion in Population II stars, discussing whether observed abundance trends are genuine signatures or artifacts of observational inaccuracies, and emphasizes the role of turbulent mixing in stellar atmospheres.

Contribution

It revisits high-quality VLT data to analyze element-specific abundance trends, highlighting the importance of turbulent mixing in interpreting atomic diffusion signatures in old stars.

Findings

Abundance trends align with atomic diffusion predictions when turbulent mixing is included.

Observed data may be affected by observational inaccuracies, complicating interpretation.

Turbulent mixing plays a crucial role in stellar surface composition evolution.

Abstract

At the "New Horizons in Globular Cluster Astronomy" conference (Padova, June 2002), two members of the VLT globular cluster team presented different views on the importance of heavy-element sedimentation in Population II stars: "The lack of evidence for depletion of Fe and Li in the atmospheres of globular cluster subgiants led some people to suspect that, for unknown reasons, Population II stars are not affected by this mechanism." (Castellani 2003) and "There should be some mechanism that prevents sedimentation." (Gratton 2003). In this review, I will argue that the scepticism behind both these statements is justified. We recently revisited the results on sedimentation in NGC 6397 stars presented by Gratton et al. (2001) using higher-quality VLT/FLAMES-UVES data (Korn et al. 2006, 2007). Element-specific abundance trends were identified which agree with atomic-diffusion predictions, if turbulent mixing below the convective envelope is accounted for in a parametrized way. Have we thus detected signatures of Atomic Diffusion in Old Stars (ADiOS)? Or are these trends mere artefacts of Conspiring Inaccuracies in Abundance Observations (CIAO)?