Some revised observational constraints on the formation and evolution of the Galactic disk

A set of 76 open clusters with abundances based upon DDO photometry and/or moderate dispersion spectroscopy has been transformed to a common metallicity scale and used to study the local structure and evolution of the galactic disk, The metallicity distribution of clusters with galactocentric distance is best described by two distinct zones, Between R-GC=6.5 and 10 kpc, the metallicity distribution has a mean [Fe/H]=0.0 and a dispersion of 0.11 dex; there is, al best, weak evidence for a shallow abundance,gradient over this distance range. Beyond R-GC=10 kpc, the metallicity distribution has a dispersion between 0.10 and 0.15 dex, but with a mean [Fe/H]=-0.3, implying a sharp discontinuity at R-GC=10 kpc. After correcting for the discontinuity, no evidence is found for a gradient perpendicular to the plane. Adopting the clusters interior to 10 kpc as a representative sample of the galactic disk over the last 7 Gyr, the cluster metallicity range is found to be approximately half that of the field star distribution. When coupled with the discontinuity in the galactocentric gradient, the discrepancy in the metallicity distribution is interpreted as an indication of significant diffusion of field stars into the solar neighborhood from beyond 10 kpc. These results imply that, contrary to earlier claims, the sun is not atypical of the stars formed in the solar circle 4.6 Gyr ago. It is suggested that the discontinuity is a reflection of the edge of the initial galactic disk as defined by the disk globular cluster system and the so-called thick disk; the initial offset in [Fe/H] created by the differences In the chemical history on either side of the discontinuity has been carried through to the current stage of galactic evolution. If correct, diffusion coupled with the absence of an abundance gradient could make the separation of field stars on the basis of galactocentric origin difficult, if not impossible. (C) 1997 The American Astronomical Society. [S0004-6256(97)02912-9].