A universal chromosome map for globular clusters: chemical calibration and environmental regulation of the multiple populations

Abstract

Chromosome maps (ChMs) are two-dimensional diagrams of UV/optical pseudocolours widely used to diagnose the multiple stellar populations (MPs) phenomenon in globular clusters. Their raw morphology is affected by the metallicity-dependent response of the photometric filters, preventing unbiased comparisons across clusters of different metallicities. We identify a cross-cluster ChM framework that accounts for this dependence, enabling an unbiased investigation of the physical drivers of MP diversity. We analyse ChMs for 23 Galactic globulars and devise a technique to correct the raw maps for the clusters' different metallicities. On the resulting "universal" ChM we define a new photometric enrichment index $S_{\rm ChM,z}$, validated against APOGEE spectroscopy. We compare this index with cluster masses, structural parameters, orbital quantities, and accretion-origin classifications. $S_{\rm ChM,z}$ correlates with the multivariate chemical abundance ranges of the enriched population and with the aluminium spread. Across the sample it increases with initial mass but correlates most strongly with a family of orbital-confinement quantities ($z_{\max}$, vertical action, apocentre, Galactocentric radius, orbital energy). The corrected ChMs provide a chemically meaningful, population-level measure of the enriched sequence. $S_{\rm ChM,z}$ does not trace a single abundance ratio but captures the cluster-to-cluster amplitude of the combined light-element variations, with particular sensitivity to the high-temperature Mg-Al/O component of proton-capture processing. Its dependence on both cluster potential depth and orbital confinement suggests that 2P chemical diversity is shaped by internal enrichment physics together with an environmental imprint, whether inherited at formation, modified by early evolution, or filtered by subsequent orbital survival.