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 tha n a few hundred million years, but the few which survive for much long er periods constitute a unique sample for probing the evolution of the galactic disk. In a CCD photometric survey far possible old open clus ters combined with previously published photometry, we have developed a list of 72 clusters of the age of the Hyades or older [Phelps et at, AJ, 107, 1079 (1994)]. Using our version of parameters based on the l uminosity difference between the main sequence turnoff and the horizon tal branch and on the color difference between the turnoff and the gia nt branch, we have calculated a ''Morphological Age Index'' (MAI) for the clusters in our list and for a sample of globular clusters. We fin d that the MAI is well correlated with the logarithm of cluster ages, as determined by fitting to theoretical isochrones. We conclude that t he index is a good measure of the relative ages of both globular and o pen clusters, although uncertainties in the models and residual metall icity effects prevent the use of the MAI as a definitive calibration o f actual cluster ages. The age distribution of the open clusters overl aps that of the globular clusters, indicating that the galactic disk b egan to develop toward the end of the period of star formation in the galactic halo. The open cluster age distribution can be fit approximat ely with a two-component exponential decay function; one component can be identified as the tail of the dominant population of thin disk ope n 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 c lusters are distributed on the galactic plane almost symmetrically abo ut 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, more than R(GC)=7.5 kpc from the galactic center; this pop ulation has a scale height of 375 pc. After accounting for the two exp onential distributions of cluster ages, there are indications of an ex cess of clusters in the age range of 5-7 Gyr; there may have been larg e bursts of star formation in that period, or perhaps a larger proport ion of the clusters forming at that time had advantageous orbits for s urvival. Either circumstance is consistent with the idea that the gala ctic disk has been repeatedly disturbed, possibly in collisions or oth er interactions with external systems, resulting in the occasional for mation of clusters with relatively large velocities perpendicular to t he 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 radi al composition gradient and the lack of a correlation between open clu ster metallicity and age found by Friel and Janes [A&A, 267, 75 (1993) ].