Dynamical friction in multi-component evolving globular clusters

TL;DR

This study investigates whether dynamical friction causes non-monotonic radial distributions of blue stragglers in globular clusters, finding that it does not and that the observed bimodality is likely due to other effects.

Contribution

The paper demonstrates through simulations that dynamical friction time remains monotonic with radius in multi-mass globular clusters, challenging previous hypotheses about bimodal distributions.

Findings

Dynamical friction time is always monotonic with radius.

Mass spectrum effects depend on radius and are dominated by the total mass density profile.

Simplified equal-mass models overestimate dynamical friction times within the half-mass radius.

Abstract

We use the Chandrasekhar formalism and direct N-body simulations to study the effect of dynamical friction on a test object only slightly more massive than the field stars, orbiting a spherically symmetric background of particles with a mass spectrum. The main goal is to verify whether the dynamical friction time (t_DF) develops a non-monotonic radial-dependence that could explain the bimodality of the Blue Straggler radial distributions observed in globular clusters. In these systems, in fact, relaxation effects lead to a mass and velocity radial segregation of the different mass components, so that mass-spectrum effects on t_DF are expected to be dependent on radius. We find that, in spite of the presence of different masses, t_DF is always a monotonic function of radius, at all evolutionary times and independently of the initial concentration of the simulated cluster. This because the radial dependence of t_DF is largely dominated by the total mass density profile of the background stars (which is monotonically decreasing with radius). Hence, a progressive temporal erosion of the BSS population at larger and larger distances from the cluster center remains the simplest and the most likely explanation of the shape of the observed BSS radial distributions, as suggested in previous works. We also confirm the theoretical expectation that approximating a multi-mass globular cluster as made of (averaged) equal-mass stars can lead to significant overestimates of t_DF within the half-mass radius.