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, F1P, is a decreasing function of the cluster present-day mass, mprst. Paper I has introduced a model mapping the region of the (mprst, F1P) 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 F1P anticorrelates more tightly with the polluted-population present-day mass, m2P,prst, than with the cluster total mass, mprst. By contrast, F1P anticorrelates poorly with the pristine-population current mass, m1P,prst. We show the loose anticorrelation between F1P and m1P,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 m2P,prst and F1P, it stems from the initially shallow relation between m2P and F1P. Clusters of the Large and Small Magellanic Clouds (LMC and SMC, respectively) appear to behave unexpectedly with respect to each other. For a given F1P, 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.