FOKKER-PLANCK MODELS AND GLOBULAR-CLUSTER EVOLUTION - THE PROBLEM OF M71

Comparisons are made 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 post-core-collapse cluster. In the standard models, the tidal boundary and the degree of mass segregation were approximately reproducible. However, we note that the inferred tidal radius, about 10 pc, is distinctly less than that based on consideration of the cluster's Galactic orbit. On the other hand, while the central surface density profiles are somewhat more concentrated than a King model, the post-core-collapse model surface density profiles are too steep to match these observations. 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 (black holes) can flatten the post-core-collapse surface brightness profile, but such models fail to reproduce the observed mass segregation and also predict an unacceptably high value for the central velocity dispersion. Models in which the heating rate is artificially enhanced do seem to be able to reproduce the observations but, in the absence of an identified source for this extra heating, such models are not physically justified. Thus, it appears that this type of Fokker-Planck model, in which post-core-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.