NGC 6397: a case study in the resolution of post-collapse globular cluster cores

Model surface brightness profiles based on Fokker–Planck simulations have been used to assess the high-resolution surface brightness profile of the globular cluster NGC 6397 given by Lauzeral et al. The profile is well fitted by the model in the maximum- expansion phase of a gravothermal oscillation with a core radius of 0.06 pc (6 arcsec), but an unresolved core cannot be ruled out. A core of this size is also expected in highly evolved clusters that include significant numbers of primordial binaries. A more massive cluster would provide better statistics, as would star counts. The greater distances of the other globular clusters and the requirements of star counting mean that very high-resolution observations will be needed to resolve cores of this size definitively.