An analytical approach to binary populations in globular clusters

TL;DR

This paper demonstrates through analytical calculations and simulations that the low binary fractions in globular clusters can be explained by the dynamical dissolution of soft binaries, assuming initial distributions similar to those in the solar neighborhood.

Contribution

It introduces an analytical model showing that dynamical dissolution of soft binaries accounts for observed binary fractions, emphasizing the role of stellar black holes and initial conditions.

Findings

Dissolution of soft binaries explains current binary fractions in GCs.

Validation of analytical estimates with N-body simulations.

Black hole dynamics significantly influence binary populations.

Abstract

Globular clusters (GCs) display much lower binary fractions than found among main-sequence stars in the solar neighborhood. The physical cause of this difference is debatable: does it reflect different star formation outcomes at low metallicity and/or high density, the dynamical processing of primordial binaries over cluster lifetimes, or a combination of the two? Starting from the assumption that the initial binary distribution in GCs is the same as the binary distribution observed in the solar neighborhood, we show with straightforward analytical calculations that the dynamical dissolution of "soft" primordial binaries can fully explain the main-sequence binary fractions in present-day GCs. We validate our estimates against a detailed N-body simulation with the Cluster Monte Carlo code. Adopting the view that the observed binary fraction in a given cluster constrains the location of the hard/soft boundary at birth, we infer that surviving Milky Way GCs had a similar distribution of birth radii to young massive clusters in the local universe. Our findings underscore the crucial role of stellar black holes (through "black hole burning") in sculpting GC binary populations and reinforce the need for realistic initial conditions in theoretical modeling of GC dynamics.