The life cycle of star clusters in a tidal field
Mark Gieles (IoA, Cambridge)

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.  We present 
model that evolves under these principles by combining the two self-similar models of 
Henon: the isolated cluster and the tidally limited cluster. The resulting model gives the 
evolution of mass and radius of initially compact clusters in a steady tidal field. 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. 
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 rho_h ~ 1/R^2. 
The remaining 2/3 are still in the first, expansion-dominated phase and their isochrones 
follow the environment-independent scaling rho_h ~ M^2; 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.