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Session 107 - Open Clusters.
Display session, Thursday, January 18
North Banquet Hall, Convention Center

[107.02] Galactic orbits and the survival of old open clusters

J. Finlay (MMO), A. Noriega-Crespo (MMO/IPAC), E. D. Friel (MMO), K. M. Cudworth (Yerkes Obs.)

Understanding the old open cluster population, which is comprised of clusters older than the Hyades, is essential to any model of the general cluster population. There are many effects which serve to explain the disruption of disk clusters, however, the unusually long survival of this old population has not yet been adequately explained. Using the galactic mass model of Allen amp; Santillan (1991, RevMxAA 22, 259) we calculate the orbits of a sample of 7 old open clusters to investigate orbital properties that would explain their survival. This mass model reproduces the galactic rotation curve and local density of the disk, and has been used by Carraro amp; Chiosi (1994, Aamp;A, 228, 751) to trace the orbits of smaller number of open clusters. Our calculations include results for two clusters, NGC 2158 and NGC 6791, whose orbits have never before been calculated. The orbits have been analyzed in terms of their filling of the zero velocity surface and the shape of their surface of section. Orbits have been calculated for a range of initial conditions which reflect the uncertainties in radial velocity, distance, and proper motion. This allows estimates of the uncertainty in any final conclusions. In addition, the orbits are compared to orbits of globular clusters, and to the field star population.

The clusters are found to occupy box orbits in regions of relatively low density, and to spend the majority of their orbital time in the corners of their orbits, at their maximum distance from the plane. NGC 2158 is found to orbit in the outer disk, and to spend over 35% of its time in a region of density less than one tenth local density, and never reaches a region of greater than half local density. NGC 6791, whose orbit takes it within 5 kpc of the galactic center, still spends over 25% of its orbit outside one tenth local density. Results for other clusters are similar. This tendency leads to fewer interactions with giant molecular clouds, which are thought to be among the main factors in the disruption of open clusters.

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