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The Galactic open cluster population offers excellent probes of the current structure of the Galactic disk and a timeline for disk evolution. Among the large population of open clusters known, the subset of very old clusters provides special insight into the early stages of disk formation and evolution.
Until recently much of what we knew about the early disk from open clusters was revealed by study of the dozen or so old clusters known within several kiloparsecs of the Sun. New surveys of potential old open clusters have enlarged this population greatly, and, in particular, have identified candidates whose ages verge on those for the youngest globular clusters. With this large sample, the age distribution of the open cluster population can be used to investigate details in the cluster formation rate over the history of the disk and to clarify the relation between the timescale for halo formation and the first appearance of the Galactic disk. The new age distribution of the open clusters and the most recent estimates for ages of the very oldest clusters from main-sequence fitting of deep photometry lead to new constraints on scenarios for the formation of the Galaxy.
Spectroscopic surveys of these newly discovered old clusters yield kinematic and abundance information that sheds light on the relationship between the old open cluster population and other disk populations, in particular the old thin disk and the thick disk. The open clusters, by comparison to the field stars, provide a complementary timeline against which to study the chemical structure and evolution of the Galactic disk. These new surveys, which span a much larger range in distance and age than previous samples, are used to define abundance gradients in the disk and the cluster age-metallicity relationship. These observations point to a complex history of chemical enrichment and mixing in the disk.
