A stellar census in globular clusters with MUSE

Metallicity spread and dispersion among first-population stars^★

¹ Institut für Astrophysik und Geophysik, Georg-August-Universität Göttingen, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany
² Astrophysics Research Institute, Liverpool John Moores University, IC2 Liverpool Science Park, 146 Brownlow Hill, Liverpool, L3 5RF, UK
³ Astronomisches Rechen-Institut, Zentrum für Astronomie der Universität Heidelberg, Mönchhofstraße 12-14, 69120 Heidelberg, Germany

^★★ Corresponding author; marilyn.latour@uni-goettingen.de

Received: 30 September 2024
Accepted: 15 January 2025

Abstract

Context. Multiple populations are ubiquitous in the old massive globular clusters (GCs) of the Milky Way. It is still unclear how they arose during the formation of a GC. The topic of iron and metallicity variations has recently attracted attention with the measurement of iron variations among the primordial population (P1) stars of Galactic GCs.

Aims. We explore the relationship between the metallicity of the P1 stars on the red-giant branch (RGB) of Galactic GCs and their ∆_F275W,F814W pseudo-color. We also measure the metallicity dispersion of P1 and P2 stars.

Methods. We used the spectra of more than 8000 RGB stars in 21 Galactic GCs observed with the integral-field spectrograph MUSE to derive individual stellar metallicities, [M/H]. For each cluster, we used Hubble Space Telescope photometric catalogs to separate the stars into two main populations (P1 and P2). We measured the metallicity spread within the primordial population of each cluster by combining our metallicity measurements with the stars’ ∆_F275W,F814W pseudo-color. We also derived metallicity dispersions (σ_[M/H]) for the P1 and P2 stars of each GC.

Results. In all but three GCs we find a significant correlation between the metallicity and the ∆_F275W,F814W pseudo-color of the P1 stars: stars with larger ∆_F275W,F814W have higher metallicities. We measure metallicity spreads that range from 0.03 to 0.24 dex and correlate with the GC masses. As for the intrinsic metallicity dispersions, when combining the P1 and P2 stars, we measure values ranging from 0.02 dex to 0.08 dex, which correlate very well with the GC masses. The two clusters that show the largest σ_[M/H] are NGC 6388 and NGC 6441. The P2 stars have metallicity dispersions that are smaller than or equal to those of the P1 stars.

Conclusions. We present a homogeneous spectroscopic characterization of the metallicities of the P1 and P2 stars in a set of 21 Galactic GCs. We find that both the metallicity spreads of the P1 stars (from the ∆_F275W,F814W spread on the chromosome maps) and the metallicity dispersions (σ_[M/H]) correlate with the GC masses, as predicted by some theoretical self-enrichment models presented in the literature.