Calcium and light-elements abundance variations from high resolution spectroscopy in globular clusters

TL;DR

This study uses high-resolution spectroscopy of 200 red giants across 17 globular clusters to investigate the relationship between calcium and light-element variations, finding minimal Ca variation and challenging previous assumptions about its role in star-to-star abundance differences.

Contribution

It provides evidence that calcium variations are negligible in most globular clusters, suggesting that Na-O anticorrelation is not directly linked to core-collapse supernova enrichment.

Findings

Calcium abundance variations are tiny (~0.02-0.03 dex) in most GCs.

Calcium and iron abundances are identical in different stellar populations within GCs.

Ca variations are not responsible for color-magnitude diagram features linked to chemical differences.

Abstract

We use abundances of Ca, O, Na, Al from high resolution UVES spectra of 200 red giants in 17 globular clusters (GCs) to investigate the correlation found by Lee et al. (2009) between chemical enrichment from SN II and star-to-star variations in light elements in GC stars. We find that (i) the [Ca/H] variations between first and second generation stars are tiny in most GCs (~0.02-0.03 dex, comparable with typical observational errors). In addition, (ii) using a large sample of red giants in M 4 with abundances from UVES spectra from Marino et al. (2008), we find that Ca and Fe abundances in the two populations of Na-poor and Na-rich stars are identical. These facts suggest that the separation seen in color-magnitude diagrams using the U band or hk index (as observed in NGC 1851 by Han et al. 2009) are not due to Ca variations. Small differences in [Ca/H] as associated to hk variations might be due to a small systematic effect in abundance analysis, because most O-poor/Na-rich (He-rich) stars have slightly larger [Fe/H] (by 0.027 dex on average, due to decreased H in the ratio) than first generation stars and are then located at redder positions in the V,hk plane. While a few GCs (M 54, omega Cen, M 22, maybe even NGC 1851) do actually show various degree of metallicity spread, our findings eliminate the need of a close link between the enrichment by core-collapse SNe with the mechanism responsible for the Na-O anticorrelation.