Homogenous abundances of Mg, Si, Ca, and Ti for about 1500 red giants in 16 globular clusters from FLAMES spectra

TL;DR

This study analyzes the abundances of Mg, Si, Ca, and Ti in about 1500 red giant stars across 16 globular clusters, revealing insights into their multiple stellar populations and chemical enrichment processes.

Contribution

It provides a large, homogeneous dataset of alpha-element abundances in globular clusters, highlighting correlations with multiple populations and cluster properties, and challenges previous discriminations between in situ and accreted clusters.

Findings

Star-to-star Si variations correlate with Na, indicating high-temperature polluters.

Mg variations are prominent in metal-poor and massive clusters.

Ca excesses suggest high-temperature proton-capture reactions in some clusters.

Abstract

The FLAMES survey ``Na-O anti-correlation and HB" uncovered the modern standard for globular clusters (GCs), that is their ubiquitous multiple stellar populations (MPs) distinct by the abundance of proton-capture elements. That survey can still be mined to extract a wealth of data. We derive new abundances of Mg, Si, Ca, and Ti for 948, 954, 1542, and 1350 red giant branch stars in 16 GCs, both formed in situ or accreted in the Milky Way. The program GCs cover the metallicity range from [Fe/H]=-2.35 dex to [Fe/H]=-0.74 dex. Both the halo and disc GCs show a clear overabundance of alpha-elements with the modulation in Mg and Si due to the MPs phenomenon in different clusters. We found star to star variations in Si abundance correlated to changes in Na in more than half of our sample, implying that temperatures in excess of about 65 MK were achieved in the polluters responsible for the enrichment. We confirm with an enlarged sample the previous result that significant variations in Mg are observed in GCs that are metal-poor, massive or both. Evidence of excess of Ca with respect to reference unpolluted field stars are found in NGC 6752 and NGC 7078, indicating the action of proton-capture reactions at very high temperature regime in these GCs. These excesses fit very well in a previously found relation as a function of a combination of cluster mass and metallicity shown by other typical signatures of MPs. At odds with previous results based on the Si abundance from APOGEE, we found that the average abundance of alpha-elements is not an efficient discriminating factor between in situ and accreted GCs.