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 0.1 dex ; there is , at 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 .