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The vast majority of open clusters persist as clusters for no more than a few hundred million years, but the few which survive for much longer periods constitute a unique sample for probing the evolution of the galactic disk. In an extensive CCD photometric survey of potential old open clusters, we have identified a number of systems that are indeed old; our data together with previously published photometry of other old open clusters, yields a catalog of 72 clusters of the age of the Hyades or older. Among the oldest open clusters are Be 17, Cr 261, NGC 6791, Be 54, and AM 2.
The age distribution of the open clusters shows a significant population up to an age of at least 10 Gyr, a distribution that overlaps the globular cluster ages. Except for a substantial excess of the youngest clusters (those with ages of less than one Gyr), the open cluster age distribution can be fit with an exponential decay model with a characteristic lifetime between 3 and 4 Gyr. The most direct interpretation of an exponentially declining age distribution is that the rate of cluster formation in the galactic (thin) disk has been nearly constant over at least the past 8 - 10 Gyr. However, theoretical models of star clusters as well as indications in the observed cluster age distribution suggest that the actual characteristic lifetime of the old clusters may be much less than a few Gyr. If so, then there is a substantial excess in the number of the oldest clusters, and there must have been a much higher rate of cluster formation in the period from 6 - 8 Gyr ago than in more recent times.
The age distribution of the open clusters shows that the galactic disk began to develop toward the end of the period of star formation in the galactic halo. The development of the disk may have included one or more major bursts of star formation, possibly caused by collisions or other interactions with external systems.
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