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Session 72 - Open Clusters.
Display session, Wednesday, January 15
Metropolitan Ballroom,

[72.04] N-Body Numerical Simulations for Orbits of Old Open Clusters

L. Hsu (MMO and Johns Hopkins Univ), A. Noriega-Crespo (MMO and IPAC), E. D. Friel (MMO and NSF)

Analysis on age distributions of open clusters has shown that the average time it takes for a cluster to be completely disrupted is a few hundred million years. However, the ages of the old open clusters are on the order of one to ten billion years. The purpose of this project was to model the clusters as n-body systems and find relations between orbital kinematics and survivability. This work builds off of an earlier project which modeled the orbits using just a center of mass, or "one-body" model . Data were available for the complete orbital parameters of seven old open clusters. These seven, along with too purely theoretical open clusters were placed in the simulations. The clusters consisted of one hundred bodies (each of one solar mass) which were placed in a model galactic potential (2). Internal interactions were taken into account, but a relatively large softening parameter was used so that binary interactions would not dominate. The results show that there is a correlation between radial distance from the galactic center and survivability. For example, the cluster ngc6791 was found to contain approximately 55% of its original mass after one gyr. The closest radial approach for ngc6791 is 4.6 kpcs. However, ngc752, which has a distance of closest radial approach of 8.7 kpcs, retained 80% of its original mass. Model clusters were identical except for center of mass positions and velocities. These results support the observation that almost all old open clusters are found at large radial distances from the center of the galaxy (usually greater than 8kpcs). Currently, data analysis is still being carried out to find trends in mass loss verses time.

[1] Finlay, Friel, Noriega-Crespo amp; Cudworth, 1995. [2] Allen amp; Santillan 1991, RevMxAA.

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