Properties of the Milky Way's Old Populations Based on Photometric
  Metallicities of the OGLE RR Lyrae Stars
  We have used photometric data on almost 91 000 fundamental-mode RR Lyrae
stars (type RRab) detected by the OGLE survey to investigate properties of old
populations in the Milky Way. Based on their metallicity distributions, we
demonstrate that the Galaxy is built from three distinct old components: halo,
bulge, and disk. The distributions reach their maxima at approximately
[Fe/H]_J95 = -1.2, -1.0, and -0.6 dex on the Jurcsik's metallicity scale,
respectively. We find that, very likely, the entire halo is formed from
infalling dwarf galaxies. It is evident that halo stars penetrate the inner
regions of the Galactic bulge. We estimate that about one-third of all RR Lyr
stars within the bulge area belong in fact to the halo population. The whole
old bulge is dominated by two populations, A and B, represented by a double
sequence in the period-amplitude (Bailey) diagram. The boundary in iron
abundance between the halo and the disk population is at about [Fe/H]_J95 =
-0.8 dex. Using Gaia DR2 for RRab stars in the disk area, we show that the
observed dispersion of proper motions along the Galactic latitude decreases
smoothly with the increasing metal content excluding a bump around [Fe/H]_J95 =
-1.0 dex.
