Mass Segregation in Globular Clusters

We present the results of a new study of mass segregation in two-componentstar clusters, based on a large number of numerical N-body simulations usingour recently developed dynamical Monte Carlo code. Specifically, we follow thedynamical evolution of clusters containing stars with individual masses m_1 aswell as a tracer population of objects with individual masses m_2=\mu m_1,using N=10^5 total stars. For heavy tracers, which could represent stellarremnants such as neutron stars or black holes in a globular cluster, wecharacterize in a variety of ways the tendency for these objects to concentratein or near the cluster core. In agreement with simple theoretical arguments, wefind that the characteristic time for this mass segregation process varies as1/\mu. For models with very light tracers (\mu <~ 10^-2), which could representfree-floating planets or brown dwarfs, we find the expected depletion of lightobjects in the cluster core, but also sometimes a significant enhancement inthe halo. Using these results we estimate the optical depth to gravitationalmicrolensing by planetary mass objects or brown dwarfs in typical globularclusters. For some initial conditions, the optical depth in the halo due tovery low-mass objects could be much greater than that of luminous stars. If weapply our results to M22, using the recent null detection of Sahu, Anderson, &King (2001), we find an upper limit of ~25% at the 63% confidence level for thecurrent mass fraction of M22 in the form of very low-mass objects.Comment: Accepted for publication in ApJ. Minor revisions reflecting the new results of Sahu et al. on M22. 13 pages in emulateapj style, including 9 figures and 3 table