Linking photometry and spectroscopy: profiling multiple populations in globular clusters

¹ Istituto Nazionale di Astrofisica – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, Padova 35122, Italy
² Dipartimento di Fisica e Astronomia “Galileo Galilei”, Univ. di Padova, Vicolo dell’Osservatorio 3, Padova 35122, Italy
³ Center for Galaxy Evolution Research and Department of Astronomy, Yonsei University, Seoul 03722, Republic of Korea
⁴ INAF – Osservatorio Astronomico di Roma, Via Frascati 33, 00040 Monte Porzio Catone, Roma, Italy
⁵ Research School of Astronomy and Astrophysics, Australian National University, Canberra, ACT 2611, Australia
⁶ South-Western Institute for Astronomy Research, Yunnan University, Kunming 650500, PR China

^★ Corresponding author; emanuele.dondoglio@inaf.it

Received: 15 November 2024
Accepted: 18 March 2025

Abstract

Our understanding of multiple populations in globular clusters (GCs) largely comes from photometry and spectroscopy. Appropriate photometric diagrams are able to disentangle first and second populations (1P and 2P, respectively), with 1P having chemical signatures similar to field stars and 2P stars showing unique light-element variations. Spectroscopy enables detailed analysis of chemical abundances in these populations. We combined multi-band photometry with extensive spectroscopic data to investigate the chemical composition of multiple populations across 38 GCs, yielding a chemical abundance dataset for stars with precise population tagging. This dataset provides the most extensive analysis to date on C, N, O, Na, Mg, and Al variations, revealing the largest sample of light-element spreads across GCs. We find that GC mass correlates with light-element variations, which supports earlier photometric studies. We investigated iron differences among 1P stars, confirming their presence in 19 GCs, and finding a spread consistent with prediction based on photometry. Notably, in eight GCs we detected a clear correlation between [Fe/H] values and their position in iron-sensitive photometric diagrams. More massive GCs display larger lithium depletion among 2P stars, which is consistent with zero at smaller masses. Some 2P stars, despite their extreme chemical differences from 1P stars, exhibit lithium abundances similar to those of 1P stars. This suggests that the polluters responsible for the 1P population have produced lithium. We analyzed anomalous stars in 10 GCs. These stars are characterized by enrichment in iron, s-process elements, and C+N+O. NGC 1851, NGC 5139 (ωCen), NGC 6656, and NGC 6715 display light-element inhomogeneities similar to 1P and 2P stars. Iron and barium enrichment varies widely, being negligible in some clusters and much larger than observational errors in others. Generally, these elemental spreads correlate with GC mass. In clusters with available data, anomalous stars show C+N+O enrichment compared to the non-anomalous stars.