Atomic Diffusion and Mixing in Old Stars. III. Analysis of NGC 6397 Stars under New Constraints

TL;DR

This study reanalyzes chemical abundances in NGC 6397 stars using new temperature calibrations, finding evolutionary abundance trends consistent with models of atomic diffusion and weak turbulent mixing, and estimating primordial lithium abundance.

Contribution

It introduces a revised temperature scale for analyzing stellar abundances and compares observations with models incorporating atomic diffusion and turbulent mixing effects.

Findings

Significant evolutionary abundance trends for Mg and Fe.

Best fit with models assuming weak turbulent mixing.

Primordial lithium abundance aligns with Big Bang predictions.

Abstract

We have previously reported on chemical abundance trends with evolutionary state in the globular cluster NGC 6397 discovered in analyses of spectra taken with FLAMES at the VLT. Here, we reinvestigate the FLAMES-UVES sample of 18 stars, ranging from just above the turnoff point (TOP) to the red giant branch below the bump. Inspired by new calibrations of the infrared flux method, we adopt a set of hotter temperature scales. Chemical abundances are determined for six elements (Li, Mg, Ca, Ti, Cr, and Fe). Signatures of cluster-internal pollution are identified and corrected for in the analysis of Mg. On the modified temperature scales, evolutionary trends in the abundances of Mg and Fe are found to be significant at the 2{\sigma} and 3{\sigma} levels, respectively. The detailed evolution of abundances for all six elements agrees with theoretical isochrones, calculated with effects of atomic diffusion and a weak to moderately strong efficiency of turbulent mixing. The age of these models is compatible with the external determination from the white dwarf cooling sequence. We find that the abundance analysis cannot be reconciled with the strong turbulent-mixing efficiency inferred elsewhere for halo field stars. A weak mixing efficiency reproduces observations best, indicating a diffusion-corrected primordial lithium abundance of log {\epsilon}(Li) = 2.57 +- 0.10. At 1.2{\sigma}, this value agrees well with WMAP-calibrated Big-Bang nucleosynthesis predictions.