Estimating the impact of the Galactic bar on the evolution of Galactic star clusters from N-body simulations

Abstract

We present an analysis of the effect of a barred gravitational potential on the evolution of several properties of star clusters, constructing a model of the Galactic bar and evolving a set of direct N-body simulations of star clusters within the resulting potential. As a first step, we have confirmed that the Galactic bar has a negligible effect on the evolution of star clusters orbiting at distances of 4 kpc or greater from the Galactic Centre. We then performed an extensive orbital analysis in order to identify typical families of planar orbits in the inner regions of a Milky Way-like barred bulge, and followed the evolution of several structural parameters of clusters belonging to the four main orbit families that were identified. We have shown that the orbit type can strongly influence the evolution of total mass, size, core radius, internal velocity dispersion and escape rates of the simulated clusters. Mass-loss rate and dissolution time of the clusters are found to be sensitive to the tidal forcing along the different orbits. We describe a method to predict dissolution times and mass-loss rates of clusters evolving within a barred potential, following any orbit and with any initial mass. Finally, we applied our method to predict the dissolution time and to reconstruct the initial mass of two Galactic bulge clusters with known orbits, namely NGC 6553 and HP 1.