The Main Sequence of Star Clusters

TL;DR

This paper introduces a new dynamical temperature-luminosity diagram to study star cluster evolution, revealing common sequences and effects of core collapse, binary fractions, and tidal conditions on cluster dissolution.

Contribution

It presents a novel dynamical temperature-luminosity framework for analyzing star cluster evolution and identifies universal behaviors post-core collapse across various initial conditions.

Findings

Clusters show a strong correlation in the dynamical temperature-luminosity diagram.

Most clusters follow a common sequence in this diagram regardless of initial conditions.

Half-mass radius remains nearly constant after core collapse until dissolution.

Abstract

A novel way of looking at the evolution of star clusters is presented. With a dynamical temperature, given by the mean kinetic energy of the cluster stars, and a dynamical luminosity, which is defined as the kinetic energy of the stars leaving the cluster in analogy to the energy of photons emitted by a star, the dissolution of star clusters is studied using a new dynamical temperature-luminosity diagram for star clusters. The investigation contains a parameter-space study of open clusters of up to N = 32768 single-mass stars with different initial density distributions, half-mass radii, tidal conditions and binary fractions. The clusters show a strong correlation between dynamical temperature and dynamical luminosity and most of the investigated cluster families share a common sequence in such a dynamical temperature-luminosity diagram. Deviations from this sequence are analyzed and discussed. After core collapse, the position of a cluster within this diagram can be defined by three parameters: the mass, the tidal conditions and the binary fraction. Due to core collapse all initial conditions are lost and the remaining stars adjust to the given tidal conditions. Binaries as internal energy sources influence this adjustment. A further finding concerns the Lagrange radii of star clusters: Throughout the investigated parameter space nearly all clusters show a constant half-mass radius for the time after core collapse until dissolution. Furthermore, the ratio of half-mass radius to tidal radius evolves onto a common sequence which only depends on the mass left in the cluster.