Deep advanced camera for surveys imaging in the globular cluster NGC 6397: Dynamical models

Abstract

We present N-body models to complement deep imaging of the metal-poor core-collapsed cluster NGC 6397 obtained with the Hubble Space Telescope. All simulations include stellar and binary evolution in step with the stellar dynamics and account for the tidal field of the Galaxy. We focus on the results of a simulation that began with 100, 000 objects (stars and binaries), 5% primordial binaries, and Population II metallicity. After 16 Gyr of evolution, the model cluster has about 20% of the stars remaining and has reached core collapse. We compare the color-magnitude diagrams of the model at this age for the central region and an outer region corresponding to the observed field of NGC 6397 (about 2-3 half-light radii from the cluster center). This demonstrates that the white dwarf (WD) population in the outer region has suffered little modification from dynamical processes - contamination of the luminosity function by binaries and WDs with non-standard evolution histories is minimal and should not significantly affect measurement of the cluster age. We also show that the binary fraction of main-sequence stars observed in the NGC 6397 field can be taken as representative of the primordial binary fraction of the cluster. For the mass function (MF) of the main-sequence stars, we find that although this has been altered significantly by dynamics over the cluster lifetime, especially in the central and outer regions, the position of the observed field is close to optimal for recovering the initial MF of the cluster stars (below the current turn-off mass). More generally we look at how the MF changes with radius in a dynamically evolved stellar cluster and suggest where the best radial position to observe the initial MF is for clusters of any age. We discuss computational constraints that restrict the N-body method to non-direct models of globular clusters currently, how this affects the interpretation of our results regarding NGC 6397, and future plans for models with increased realism.