High Precision Differential Abundance Measurements in Globular Clusters: Chemical Inhomogeneities in NGC 6752

TL;DR

This study uses high-precision differential abundance analysis of globular cluster NGC 6752's stars, revealing small but significant chemical inhomogeneities and correlations among elements, suggesting complex chemical evolution.

Contribution

First high-precision differential abundance measurements in NGC 6752, uncovering subtle chemical variations and correlations not previously detected.

Findings

Abundance dispersions are twice the measurement uncertainty.

Strong positive correlations between all elements and Na.

Variations suggest inhomogeneous chemical evolution or He variations.

Abstract

We report on a strictly differential line-by-line analysis of high quality UVES spectra of bright giants in the metal-poor globular cluster NGC 6752. We achieved high precision differential chemical abundance measurements for Fe, Na, Si, Ca, Ti, Cr, Ni, Zn, Y, Zr, Ba, La, Ce, Pr, Nd, Sm, Eu and Dy with uncertainties as low as ~0.01 dex (~2%). We obtained the following main results. (1) The observed abundance dispersions are a factor of ~2 larger than the average measurement uncertainty. (2) There are positive correlations, of high statistical significance, between all elements and Na. (3) For any pair of elements, there are positive correlations of high statistical significance, although the amplitudes of the abundance variations are small. Removing abundance trends with effective temperature and/or using a different pair of reference stars does not alter these results. These abundance variations and correlations may reflect a combination of (a) He abundance variations and (b) inhomogeneous chemical evolution in the pre- or proto-cluster environment. Regarding the former, the current constraints on Delta Y from photometry likely preclude He as being the sole explanation. Regarding the latter, the nucleosynthetic source(s) must have synthesised Na, alpha, Fe-peak and neutron-capture elements and in constant amounts for species heavier than Si; no individual object can achieve such nucleosynthesis. We speculate that other, if not all, globular clusters may exhibit comparable abundance variations and correlations to NGC 6752 if subjected to a similarly precise analysis.