Atomic Diffusion and Mixing in Old Stars V: A deeper look into the Globular Cluster NGC 6752

TL;DR

This study investigates atomic diffusion and mixing effects on elemental abundances in NGC 6752, revealing weak but consistent abundance trends across stellar populations that inform models of stellar structure and primordial element levels.

Contribution

It provides new observational evidence of atomic diffusion and mixing in globular cluster stars, refining models of stellar evolution and initial lithium abundance.

Findings

Weak abundance trends for Ca, Ti, and Fe with evolutionary phase.

Identification of three distinct stellar populations in NGC 6752.

Derived initial lithium abundance slightly below standard BBN predictions.

Abstract

Abundance trends in heavier elements with evolutionary phase have been shown to exist in the globular cluster NGC 6752 [Fe/H]=-1.6. These trends are a result of atomic diffusion and additional (non-convective) mixing. Studying such trends can provide us with important constraints on the extent to which diffusion modifies the internal structure and surface abundances of solar-type, metal-poor stars. Taking advantage of a larger data sample, we investigate the reality and the size of these abundance trends and address questions and potential biases associated with the various stellar populations that make up NGC6752. Based on uvby Str\"omgren photometry, we are able to separate three stellar populations in NGC 6752 along the evolutionary sequence from the base of the red giant branch down to the turnoff point. We find weak systematic abundance trends with evolutionary phase for Ca, Ti, and Fe which are best explained by stellar-structure models including atomic diffusion with efficient additional mixing. We derive a new value for the initial lithium abundance of NGC 6752 after correcting for the effect of atomic diffusion and additional mixing which falls slightly below the predicted standard BBN value. We find three stellar populations by combining photometric and spectroscopic data of 194 stars in the globular cluster NGC 6752. Abundance trends for groups of elements, differently affected by atomic diffusion and additional mixing, are identified. Although the statistical significance of the individual trends is weak, they all support the notion that atomic diffusion is operational along the evolutionary sequence of NGC 6752.