The main sequence of star clusters

Abstract

ABSTRACT A novel way of looking at the evolution of star clusters is presented. With a dynamicaltemperature, given by the mean kinetic energy of the cluster stars, and a dynamicalluminosity, which is defined as the kinetic energy of the stars leaving the cluster inanalogy to the energy of photons emitted by a star, the dissolution of star clusters isstudied using a new dynamical temperature-luminosity diagram for star clusters. Theinvestigation contains a parameter-space study of open clusters of up to N = 32768single-mass stars with different initial density distributions, half-mass radii, tidal con-ditions and binary fractions. The clusters show a strong correlation between dynamicaltemperature and dynamical luminosity and most of the investigated cluster familiesshare a common sequence in such a dynamical temperature-luminosity diagram. Devi-ations from this sequence are analyzed and discussed. After core collapse, the positionof a cluster within this diagram can be defined by three parameters: the mass, thetidal conditions and the binary fraction. Due to core collapse all initial conditions arelost and the remaining stars adjust to the given tidal conditions. Binaries as internalenergy sourcesinfluence this adjustment. A further finding concerns the Lagrangeradiiof star clusters: Throughout the investigated parameter space nearly all clusters showa constant half-mass radius for the time after core collapse until dissolution. Further-more, the ratio of half-mass radius to tidal radius evolves onto a common sequencewhich only depends on the mass left in the cluster.Keywords: stellar dynamics – galaxies: star clusters – methods: N-body simulations– Hertzsprung–Russel (HR) diagram – open clusters and assoziations: general