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Session 82 - Globular Clusters.
Display session, Wednesday, January 17
North Banquet Hall, Convention Center
The sparse globular cluster, Pal 5, is likely to be destroyed by tidal stresses the next time it crosses the Galactic plane. We study this problem, treating Pal 5 as a self-consistent dynamical system orbiting through an external potential that represents the Galaxy, using the cluster's observed galactocentric position and velocity as initial conditions for the orbit. This study is motivated in part by recent observational and theoretical work on multiple cores at the centers of galaxies. Possible progenitors of these cores are star clusters or merger remnants. Understanding the dynamical fate of progenitors can best be addressed by such experiments. We have adapted methods used some time ago to study the survival of a galaxy in a cluster to this case.
Experiments have been run with cluster M/L=1, 3, and 10. The cluster blows up shortly after passing through the Galactic plane at M/L=1, it survives (albeit with fairly heavy damage; 25% of the initial mass was lost) at M/L=3, and it comes through pretty well unscathed at M/L=10. We haven't tried to trap the mass limits more closely than that.
The intermediate case, M/L = 3, provides an excellent example for a detailed dynamical study of the destruction process and of the preferential stripping of low-mass stars. Trajectories and velocity increments given to particles that are stripped out of the cluster provide a sharp tool to probe these processes. The fundamental dynamics is similar to damage during the close approach phase of galaxy collisions: most of the particles that escape are moving toward the center of the cluster (relative to the cluster) at the time the cluster crosses the Galactic plane. Strong tidal stresses at the moment of crossing give them an additional velocity increment sufficient to remove them from the cluster. Their velocity is further boosted by self-consistent dynamical processes within the cluster as the cluster responds globally to the tidal impulse. Most of the ejected material streams along the orbit, so debris from earlier crossings stretches out along the cluster's orbit.
Program listing for Wednesday