Simulations of Globular Cluster Evolution with Multiple Stellar Populations

TL;DR

The paper addresses the origin and dynamical evolution of multiple stellar populations in globular clusters within the AGB formation framework. It deploys about 200 MOCCA simulations that incorporate gas re-accretion, external environment, and MSP formation timing to reproduce Milky Way GC observables, notably the $N_2/N_{tot}$ ratio, $R_h$, and cluster mass, while highlighting the persistent challenge of the mass–$N_2/N_{tot}$ correlation. A speculative extension to the AGB scenario is proposed to account for this correlation by linking gas availability and cluster migration to the evolving galactic environment, though this requires several physically coordinated conditions and remains unconfirmed. The study also investigates spatial and kinematic signatures, such as transient 1P overconcentration of RGB stars in BH-containing clusters, emphasizing the need for MS-star observations to robustly test MSP distributions and the necessity of detailed modeling of the Galaxy’s early tidal field.

Abstract

The formation of stars with light-element abundance variations in globular clusters and the subsequent dynamical evolution of these multiple populations remains an open question. One of the most widely discussed is the AGB scenario, in which chemically processed material from the envelopes of AGB stars mixes with re-accreted primordial gas flowing into the center of the cluster. Based on this scenario, more than two hundred MOCCA simulations of cluster evolution have been carried out, incorporating additional physical processes related to the external environment of globular clusters and the initial properties of multiple stellar populations. Analysis of the simulations shows that most observed properties of multiple stellar populations and the global parameters of Milky Way clusters are well reproduced, with the exception of the correlation between cluster mass and the fraction of second-population stars. We present a speculative scenario of globular cluster evolution that may account for the observed properties of Milky Way clusters, including the correlation between cluster mass and the fraction of enriched stars. The scenario further predicts that, under certain conditions, the pristine first population can be more centrally concentrated than the enriched second population, as observed in some clusters. \end{abstract

Figures (6)

• Figure 1: Left panel: Evolution of $\mathrm{R_h}$ for nTD-MSP model, and for TD-MSP model, and evolution of the total cluster mass for nTD-MSP model, and TD-MSP model. Right panel: Evolution of the ratio $\mathrm{N_2/N_{tot}}$ for the nTD-MSP model, and TD-MSP model. The global cluster parameters are listed at the top of each panel: $\mathrm{N_1}$ - number of 1P objects, $\mathrm{N_2}$ - number of 2P objects, $\mathrm{Z}$ - metallicity, $\mathrm{R_g}$ - galactocentric distance (size of the circular orbit), TF - tidally filling 1P , $\mathrm{W_{o1}}$ – King parameter for 1P , $\mathrm{conc}$ - concentration parameter ($\mathrm{conc_{pop}=R_{h2}/R_{h1}}$), $\mathrm{bf}$ – binary fraction, $\mathrm{Q_1}$ - virial ratio for 1P .
• Figure 2: Left panel: Evolution of $\mathrm{R_h}$ for nTD-MSP model, TD-MSP model, and TD-MSP model with migration, and evolution of the total cluster mass for nTD-MSP model, TD-MSP model, and TD-MSP model with migration. Right panel: Evolution of the ratio $\mathrm{N_2/N_{tot}}$ for nTD-MSP model, TD-MSP model, and TD-MSP with migration model. The global cluster parameters are listed at the top of each panel and are the same as for Figure \ref{['f:Fig1']}.
• Figure 3: Left panel: Evolution of $\mathrm{R_h}$ and the cluster total mass for TD-MSP models with different $\mathrm{Q_1}$. Right panel: Evolution of the ratio $\mathrm{N_2/N_{tot}}$ for TD-MSP models with different $\mathrm{Q_1}$. The global cluster parameters are listed at the top of each panel and are the same as for Figure \ref{['f:Fig1']}.
• Figure 4: Left panel: Evolution of $\mathrm{R_h}$ and the cluster total mass for TD-MSP models with a different King parameter, $\mathrm{W_{o1}}$. Right panel: Evolution of the ratio $\mathrm{N_2/N_{tot}}$ for TD-MSP models with a different King parameter, $\mathrm{W_{o1}}$. The global cluster parameters are listed at the top of each panel and are the same as for Figure \ref{['f:Fig1']}.
• Figure 5: Left panel: Evolution of $\mathrm{R_h}$ and the cluster total mass for TD-MSP models with different galactocentric distance ($\mathrm{R_g}$). Right panel: Evolution of the ratio $\mathrm{N_2/N_{tot}}$ for TD-MSP models different ${R_g}$. The global cluster parameters are listed at the top of each panel and are the same as for Figure \ref{['f:Fig1']}.