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Session 107 - Open Clusters.
Display session, Thursday, January 18
North Banquet Hall, Convention Center
Theories about Galactic disk evolution can be evaluated by comparing their predictions to the old open cluster system. The ``inside-out'' type of theory predicts that the ISM in the inner part of the Galactic disk is older and more chemically evolved than that of the outer part. Recent efforts by many other researchers have resulted in a sample of open clusters that is, for the first time, large enough for statistically significant testing. From the 47 old and well-studied open clusters in the literature, I arbitrarily defined outer-disk clusters as those with Galactocentric radii greater than 9.5 kpc. The Student's t paired test applied to 13 pairs of clusters indicates that a cluster in the inner disk has a statistically-significant higher [Fe/H] than one formed at the same time in the outer disk. This was consistent with the results of a Student's t mean test applied to the total sample of 47 clusters. This showed significantly different mean [Fe/H] values in the sense that the inner disk was higher than the outer disk. The inner and outer disk clusters had similar mean ages. The first two results are in accordance with inside-out theories: the latter is not. A qualitative examination of the sample shows that the mean age result can not be attributed to the preferential destruction of inner-disk open clusters, but rather to a lack of young outer-disk clusters.
Further, the subsystem of extremely old open clusters (\sim5--7 Gyr old) extends to a larger radial distance than does the younger subsystem. If open clusters remain approximately at the radial position at which they were formed, this result is also inconsistent with inside-out theories. If one instead assumes inside-out disk formation, then this result could be interpreted as radial expansion of the open cluster system.
