The Galactic System of Old Star Clusters: The Development of the Galactic Disk

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 a charge coupled device (CCD) photometric survey for possible old open clusters combined with previously published photometry, we have developed a list of 72 clusters of the age of the Hyades or older (Phelps (1994). Using our version of parameters based on the luminosity difference between the main sequence turnoff and the horizontal branch and on the color difference between the turnoff and the giant branch, we have calculated a 'Morphological Age Index' (MAI) for the clusters in our list and for a sample of globular clusters. We find that the MAI is well correlated with the logarithm of cluster ages, as determined by fitting to theoretical isochrones. We conclude that the index is a good measure of the relative ages of both globular and open clusters, although uncertainties in the models and residual metallicity effects prevent the use of the MAI as a definitive calibration of actual cluster ages. The age distribution of the open clusters overlaps that of the globular clusters, indicating that the galactic disk began to develop toward the end of the period of star formation in the galactic halo. The open cluster age distribution can be fit approximately with a two-component exponential decay function; one component can be identified as the tail of the dominant population of thin disk open clusters with an age scale factor of 200 Myr, and the other consists of longer-lived clusters with an age scale of 4 Gyr. The young open clusters are distributed on the galactic plane almost symmetrically about the Sun with a scale height perpendicular to the galactic plane of 55 pc. The old population consists of rich clusters found only in the outer disk, nore than R_GC = 7.5 kpc from the galactic center; this population has a scale height of 375 pc. After accounting for the two exponential distributions of cluster ages, there are indications of an excess of clusters in the age range of 5-7 Gyr; there may have been large bursts of star formation in that period, or perhaps a larger proportion of the clusters forming at that time had advantageous orbits for survival. Either circumstance is consistent with the idea that the galactic disk has been repeatedly disturbed, possibly in collisions or other interactions with external systems, resulting in the occasional formation of clusters with relatively large velocities perpendicular to the plane; these are the clusters that have survived until the present. Finally, the repeated accretion of low angular momentum material onto the disk from the halo or beyond would also explain the observed radial composition gradient and the lack of a correlation between open cluster metallicity and age found by Friel & Janes (1993).