An ACS Deep Field in the Globular Cluster NGC 6397: Dynamical Models

We present N-body models to complement ACS imaging of the metal-poor core-collapsed cluster NGC 6397 during HST Cycle 13 (see poster by Richer et al. at this meeting). We focus on the results of a simulation that began with 100,000 stars, 5% primordial binaries and Population II metallicity. After 15 Gyr of evolution the model cluster has about 20% of the stars remaining and is core-collapsed. We find that in the region of the model corresponding to the observed field of NGC 6397 (about two half-mass radii from the cluster center) the white dwarf population has sufferred little modification from dynamical processes -- contamination of the luminosity function by binaries and white dwarfs with non-standard evolution histories is minimal and does not affect measurement of the cluster age. For the luminosity function 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, that the position of the ACS field is optimal for recovering the initial mass function of the cluster stars. We compare the color-magnitude diagrams of the model and cluster and discuss the implications for the binary fraction of NGC 6397 and the subsequent evolution of the cluster. We have also performed simulations starting with 100,000 stars and 10% binaries as well as 200,000 stars with 3% binaries. All simulations include stellar and binary evolution and account for the tidal field of the Galaxy.