Kinematics of RR Lyrae stars in the Galactic bulge with OGLE-IV and Gaia
  DR2
  We analyze the kinematics and spatial distribution of 15,599 fundamental-mode
RR Lyrae (RRL) stars in the Milky Way bulge by combining OGLE-IV photometric
data and Gaia DR2 proper motions. We show that the longitudinal proper motions
and the line-of-sight velocities can give similar results for the rotation in
the Galactic central regions. The angular velocity of bulge RRLs is found to be
around $35$ km s$^{-1}$ kpc$^{-1}$, significantly smaller than that for the
majority of bulge stars ($50-60$ km s$^{-1}$ kpc$^{-1}$); bulge RRLs have
larger velocity dispersion (120$-$140 km s$^{-1}$) than younger stars. The
dependence of the kinematics of the bulge RRLs on their metallicities is shown
by their rotation curves and spatial distributions. Metal-poor RRLs
([Fe/H]<$-1$) show a smaller bar angle than metal-rich ones. We also find clues
suggesting that RRLs in the bulge are not dominated by halo stars. These
results might explain some previous conflicting results over bulge RRLs and
help understand the chemodynamical evolution of the Galactic bulge.
