Tracing ωCentauri’s origins: Spatial and chemical signatures of its formation history

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 Dipartimento di Tecnica e Gestione dei Sistemi Industriali, Università degli Studi di Padova, Stradella S. Nicola 3, 36100 Vicenza, Italy
⁴ South-Western Institute for Astronomy Research, Yunnan University, Kunming 650500, PR China

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 22 July 2025
Accepted: 20 September 2025

Abstract

ωCentauri (ωCen), with its unmatched chemical complexity, is the most enigmatic Galactic globular cluster (GC). We combine photometric and spectroscopic catalogs to identify distinct stellar populations within ωCen, investigate their spatial distribution and chemical properties, and uncover new insights into the cluster’s formation history. Our population tagging identified the iron-poor stars commonly found in most GCs: the first population (1P), with halo-like chemical composition, and the second population (2P), enriched in elements produced by proton-capture processes. Similarly, we divided the iron-rich stars (the anomalous stars) into two groups, AI and AII, which exhibit light-element abundance distributions similar to 1P and 2P stars, respectively. The wide radial extension of our dataset (five times the half-light radius) allowed us, for the first time, to directly and unambiguously compare the fractions of these populations at different radii. We find that the 2P and AII stars are more centrally concentrated than the 1P and AI stars. The remarkable similarities between the 1P-2P and AI-AII radial distributions strongly suggest that these two groups of stars originated through similar mechanisms. Our chemical analysis indicates that the 1P and AI stars (the lower stream) developed their inhomogeneities through self-enrichment from core-collapse supernovae (and possibly ejecta from other massive stars), as supported by their increasing α-element abundances with [Fe/H]. These populations contributed proton-capture-processed material to the intracluster medium, from which the chemically extreme 2P and AII stars (the upper stream) formed. Additional polluters, such as intermediate-mass asymptotic giant branch stars and Type Ia supernovae, likely played a role in shaping the AII population. Finally, we propose that 2P and AII stars with intermediate light-element abundances (the middle stream) formed via dilution between the pure ejecta that created the upper stream and the lower-stream material.