On the Metallicity Gradient in the Galactic Disk
  The problem of the chemical composition gradient in the galactic disk is
studied based on a sample of metallicity estimates of open star clusters, using
Gaia DR2-improved distance estimates. A clearly non-monotonic variation was
observed in the average metallicity of clusters with increasing galactocentric
distance. One can clearly see the metallicity jump of 0.22 in [Fe/H] at a
Galactocentric distance of about 9.5 kpc, which appears to be linked to the
outer boundary of the thinnest and youngest component of the galactic disk. The
absence of a significant metallicity gradient in the internal (R<9 kpc) and
external (R>10 kpc) regions of the disk demonstrates the absence of noticeable
metal enrichment at times of the order of the ages corresponding to those of
the disk regions under consideration. Observational data show that the disk
experiences noticeable metal enrichment only during the starburst epochs. No
significant dependence was found between the average metallicity and the age of
the clusters.
