Metallicity field and selection effects in spatial distribution of the
  Galactic globular cluster system
  The prospects for using the present-day data on metallicity of globular
clusters (GCs) of the Galaxy to put constraints on the distance to the Galactic
center, $R_0$, are considered. We have found that the GCs of the metal-rich and
metal-poor subsystems separately form a bar-like structure in metallicity maps
whose parameters are very close to those for the Galactic bar. The results
indicate the existence of a bar component within both the metal-rich and
metal-poor subsystems of GCs. The bar GCs could have formed within the already
existing Galactic bar or could have later been locked in resonance with the
bar. We conclude that substantial constraints on the $R_0$ value can be
obtained only with non-axisymmetric models for the space distribution of GC
metallicities with the allowance for the subdivision of GCs into subsystems. We
found evidence for a bar extinction component that causes the observational
incompleteness of GCs in the far side of the Galactic bar and in the
"post-central" region. This selection effect should be taken into account when
determining $R_0$ from the spatial distribution of GCs.
