Cracking the relation between mass and 1P-star fraction of globular clusters: I. Present-day cluster masses as a first tool

Abstract

The phenomenon of multiple stellar populations is exacerbated in massive globular clusters, with the fraction of first-population (1P) stars a decreasing function of the cluster present-day mass. We decipher this relation in far greater detail than has been done so far. We assume (i) a fixed stellar mass threshold for the formation of second-population (2P) stars, (ii) a power-law scaling $F_{1P} \propto m_{ecl}^{-1}$ between the mass $m_{ecl}$ of newly-formed clusters and their 1P-star fraction $F_{1P}$, and (iii) a constant $F_{1P}$ over time. The $F_{1P}(m_{ecl})$ relation is then evolved up to an age of 12Gyr for tidal field strengths representative of the entire Galactic halo. The 12Gyr-old model tracks cover extremely well the present-day distribution of Galactic globular clusters in (mass,$F_{1P}$) space. The distribution is curtailed on its top-right side by the scarcity of clusters at large Galactocentric distances, and on its bottom-left side by the initial scarcity of very high-mass clusters, and dynamical friction. Given their distinct dissolution rates, "inner" and "outer" model clusters are offset from each other, as observed. The locus of Magellanic Clouds clusters in (mass,$F_{1P}$) space is as expected for intermediate-age clusters evolving in a gentle tidal field. Given the assumed constancy of $F_{1P}$, we conclude that 2P-stars do not necessarily form centrally-concentrated. We infer a minimum mass of $4 \cdot 10^5~M_{\odot}$ for multiple-populations clusters at secular evolution onset. This high-mass threshold severely limits the amount of 2P-stars lost from evolving clusters, thereby fitting the low 2P-star fraction of the Galactic halo field.