2011/01/31 by Mark Gieles, Douglas C. Heggie, HongSheng Zhao · 1 citation
Physics and Astronomy · #astro-ph.GA #astro-ph.SR
paper · pdf · doi:10.1111/j.1365-2966.2011.18320.x
published as Mon. Not. R. Astron. Soc. 413, 2509-2524 (2011) · Updated to match final journal style
arxiv created 2011/06/16 · arxiv updated 2011/06/17
The evolution of globular clusters due to 2-body relaxation results in an outward flow of energy and at some stage all clusters need a central energy source to sustain their evolution. Henon provided the insight that we do not need to know the details of the energy production in order to understand the relaxation-driven evolution of the cluster, at least outside the core. He provided two self-similar solutions for the evolution of clusters based on the view that the cluster as a whole determines the amount of energy that is produced in the core: steady expansion for isolated clusters, and homologous contraction for clusters evaporating in a tidal field. We combine these models: the half-mass radius increases during the first half of the evolution, and decreases in the second half; while the escape rate approaches a constant value set by the tidal field. We refer to these phases as `expansion dominated' and `evaporation dominated'. These simple analytical solutions immediately allow us to construct evolutionary tracks and isochrones in terms of cluster half-mass density, cluster mass and galacto-centric radius. From a comparison to the Milky Way globular clusters we find that roughly 1/3 of them are in the second, evaporation-dominated phase and for these clusters the density inside the half-mass radius varies with the galactocentric distance R as rhoh ~ 1/R2. The remaining 2/3 are still in the first, expansion-dominated phase and their isochrones follow the environment-independent scaling rhoh ~ M2; that is, a constant relaxation time-scale. We find substantial agreement between Milky Way globular cluster parameters and the isochrones, which suggests that there is, as Henon suggested, a balance between the flow of energy and the central energy production for almost all globular clusters.