New parameters for star cluster dynamics: observational results

TL;DR

This paper introduces new empirical parameters derived from HST observations to quantify the internal dynamical evolution of globular clusters, confirming their effectiveness through correlations with established dynamical indicators.

Contribution

The study applies previously defined parameters to a large sample of Galactic GCs, demonstrating their correlation with cluster dynamical age and evolution stages.

Findings

A5 and P5 are highest in the most dynamically evolved GCs.

The parameters strongly correlate with the A+rh dynamical age indicator.

Results support the parameters' effectiveness in tracing cluster dynamical evolution.

Abstract

We recently used a large set of Monte Carlo simulations of globular clusters (GCs) to define new fully empirical parameters (named A5, P5, and S2.5) able to trace the internal dynamical evolution of dense stellar systems. These parameters are specifically designed to quantify the steepness of the cumulative radial distribution of stars in the innermost region of the host system, which tends to progressively increase with dynamical aging due to core contraction. Following the original definitions, here we measure A5 and P5 in a sample of 40 Galactic GCs homogeneously surveyed through HST photometric observations. In agreement with the predictions of our simulations, the largest values of A5 and P5 are found for the most dynamically evolved GCs, i.e., those previously classified as post-core collapse systems based on the shape of their density profile, and those characterized by the shortest central relaxation times. Moreover, the new dynamical parameters here measured strongly correlate with A+rh, another fully empirical, independent parameter that traces the dynamical age of star clusters through the level of central segregation of blue straggler stars.