Fokker-Planck Models and Globular Cluster Evolution: The Problem of M71

Results are presented of comparisons between star-count mass functions and surface density profiles for the globular cluster M71 and multimass, tidally truncated Fokker-Planck simulations reheated by three-body binaries. The degree of mass segregation and the short relaxation time observed for the cluster suggest that M71 should be a postcore-collapse cluster. It is shown that gravothermal oscillations are unlikely to affect the comparison between observations and theory except in the case of clusters with extreme cusps. The presence of massive stellar remnants can flatten the postcore-collapse surface brightness profile, but such models fail to reproduce the observed mass segregation and also predict an unacceptably high value of the central velocity dispersion. Models in which the heating rate is artificially enhanced are able to reproduce the observations, but, in the absence of an identified source for this extra heating, such models are not physically justified. It is argued that this type of Fokker-Planck model, in which postcore-collapse expansion is driven solely by three-body binaries, is incomplete and that additional physics, such as the effects of stellar evolution or primordial binaries, is required.