Effects of External Tidal Field on the Evolution of Multi-mass Star Clusters

We present N-body simulations of realistic globular clusters containing initial mass function in the galaxy to study effects of tidal field systematically on the properties of outer parts of globular clusters. Using NBODY6 which takes into account the two-body relaxation correctly, we investigate general evolution of globular clusters in galactic tidal field. For simplicity, we have employed only spherical components (bulge and halo) of the galaxy. Total number of stars in our simulations was about 20,000. All simulations were done for several orbital periods in order to understand the development of the tidal tails. In our scaled down models, the relaxation time is sufficiently short to show the mass segregation effect, but we did not go far enough to see the core-collapse, and the fraction of stars lost from the cluster at the end of simulation is only about ∼ 10%. The radial distribution of extra-tidal stars can be described by a power law with a slope around −3. The direction of tidal tails are determined by the orbits and locations of the clusters. We find that the length of tidal tails increases towards the apogalacticon and decreases towards the perigalacticon. This is an anti-correlation with the strength of the tidal field, caused by the fact that the the time-scale for the stars to respond to the potential is similar to the orbital time-scale of the cluster. When the length of tidal tails decreases some of the stars in the tidal tails are recaptured by the host cluster. From the investigation of velocity anisotropy of the model clusters, we find that in the early stages of globular cluster evolution the clusters have radial anisotropy in the outermost parts, while clusters are nearly isotropic in the cental region. The radial anisotropy decreases with time.