Cracking the Relation between Mass and 1P-star Fraction of Globular Clusters. II. The Masses in 1P and 2P Stars as a Second Tool

Abstract

Galactic globular clusters contain two main groups of stars, the pristine or 1P stars, and the polluted or 2P stars. The pristine-star fraction in clusters, F 1P , is a decreasing function of the cluster present-day mass, m prst. Paper I has introduced a model mapping the region of the (m prst, F 1P ) space occupied by clusters, with the cluster mass threshold for 2P-star formation a key building block. We now expand this model to the pristine-star fraction in dependence of the pristine- and polluted-population masses. A. P. Milone et al. found that F 1P anticorrelates more tightly with the polluted-population present-day mass, m 2P,prst, than with the cluster total mass, m prst. By contrast, F 1P anticorrelates poorly with the pristine-population current mass, m 1P,prst. We show the loose anticorrelation between F 1P and m 1P,prst to result from a roughly constant pristine-population mass in clusters as they start their long-term evolution in the Galactic tidal field. As for the tight anticorrelation between m 2P,prst and F 1P , it stems from the initially shallow relation between m 2P and F 1P . Clusters of the Large and Small Magellanic Clouds (LMC and SMC, respectively) appear to behave unexpectedly with respect to each other. For a given F 1P , LMC clusters are more massive than SMC clusters despite their enduring a stronger tidal field. This is opposite to how the Galactic outer- and inner-halo clusters behave. The explanation may lie in cluster formation conditions. Finally, we wonder whether the single-population clusters NGC 419 and Rup 106 formed as multiple-population clusters.