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

Eugenio Carretta

TL;DR

The paper presents homogeneous Mg, Si, Ca, and Ti abundances for about 1500 RGB stars across 16 Galactic globular clusters using FLAMES data, spanning [Fe/H] from -2.35 to -0.74. It confirms a broadly constant alpha-element pattern among clusters and finds Si variations correlated with Na in many clusters, indicating MPs with temperatures above ~65 MK; Mg variations are prominent only in metal-poor or massive clusters, and Ca excesses appear in a couple of clusters at very high temperatures. The authors compare their optical abundances with Gaia-ESO, GALAH, and APOGEE, finding general agreement but notable survey-dependent offsets and scatter, and they show that alpha-element abundances do not robustly discriminate in situ versus accreted clusters. The results support a scenario where MPs produce limited changes in average alpha-element abundances, while high-temperature proton-capture processing leaves detectable signatures (e.g., Ca excess) that correlate with cluster mass and metallicity, providing stringent constraints for GC formation and enrichment models and serving as a benchmark for future surveys like 4MOST and WEAVE.

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.

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

TL;DR

The paper presents homogeneous Mg, Si, Ca, and Ti abundances for about 1500 RGB stars across 16 Galactic globular clusters using FLAMES data, spanning [Fe/H] from -2.35 to -0.74. It confirms a broadly constant alpha-element pattern among clusters and finds Si variations correlated with Na in many clusters, indicating MPs with temperatures above ~65 MK; Mg variations are prominent only in metal-poor or massive clusters, and Ca excesses appear in a couple of clusters at very high temperatures. The authors compare their optical abundances with Gaia-ESO, GALAH, and APOGEE, finding general agreement but notable survey-dependent offsets and scatter, and they show that alpha-element abundances do not robustly discriminate in situ versus accreted clusters. The results support a scenario where MPs produce limited changes in average alpha-element abundances, while high-temperature proton-capture processing leaves detectable signatures (e.g., Ca excess) that correlate with cluster mass and metallicity, providing stringent constraints for GC formation and enrichment models and serving as a benchmark for future surveys like 4MOST and WEAVE.

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.
Paper Structure (12 sections, 16 figures, 5 tables)

This paper contains 12 sections, 16 figures, 5 tables.

Table of Contents

  1. Introduction
  2. Analysis
  3. Error budget
  4. Comparison with other spectroscopic surveys
  5. Results: the general pattern
  6. Temperature tomography of multiple populations
  7. The high temperature regime: Mg and Si
  8. The very high temperature regime: Ca
  9. Discussion and summary
  10. Abundances as a function of effective temperature
  11. Tables of sensitivity and internal errors
  12. Comparison with the GES, APOGEE, and GALAH surveys

Figures (16)

  • Figure 1: Fractions of stars with primordial (P, red circles), intermediate (I, blue circles), and extreme (E, green circles) composition in our program GCs, superimposed to a reference sample of unpolluted field stars from Carretta (2013: grey open triangles).
  • Figure 2: Average abundance ratios [Mg/Fe], [Si/Fe], [Ca/Fe], [Ti/Fe] i as a function of metallicity [Fe/H], with the 1$\sigma$ spread represented as error bars. Filled symbols are for the 16 GCs of the present work, empty symbols indicates GCs from previous, individual studies. Red and blue colour are for in situ and ex situ GCs, respectively. Grey points are field stars from Gratton et al. (2003).
  • Figure 3: Differences of the average [Si/Fe] abundance from this work (extended sample) and APOGEE (from Horta et al. 2020) for 20 GCs in common. Symbols are as in Fig. \ref{['f:mediepfield']}.
  • Figure 4: Abundance ratios [Mg/Fe], [Si/Fe], and [Ca/Fe] as a function of the orbital energy of the GCs, from Savino and Posti (2019). Red and blue symbols are for GCs formed in situ and associated to Gaia-Enceladus. Filled and empty circles refer to the present and previous works, respectively. In each panel, the abundance level of NGC 288 is indicated by the vertical line.
  • Figure 5: Average abundance ratios [Mg/Fe] and [Si/Fe] for GCs in our extended sample, using only the unpolluted P fraction (left panel), the polluted component with intermediate I and extreme E composition (middle and right panel, respectively) in each GC. The horizontal lines trace by eye the upper envelope of abundances in the left panel and are reproduced in the other two panels.
  • ...and 11 more figures