Galactic Rotation from Cepheids with Gaia DR2 and Effects of
  Non-Axisymmetry
  We apply a simple axisymmetric disc model to 218 Galactic Cepheids whose
accurate measurements of the distance and velocities are obtained by
cross-matching an existing Cepheids catalogue with the Gaia DR2 data. Our model
fit determines the "local centrifugal speed", $V_\mathrm{c}$ $-$ defined as the
rotation speed required to balance the local radial gravitational force $-$ at
the Sun's location to be $V_{c}(R_0)=236\pm 3$ km s$^{-1}$ and the Sun's
azimuthal and radial peculiar motions to be $V_{\odot}=12.4\pm0.7$ km s$^{-1}$
and $U_{\odot}=7.7\pm0.9$ km s$^{-1}$, respectively. These results are obtained
with strong priors on the solar radius, $R_0=8.2\pm0.1$ kpc, and Sun's angular
rotation velocity, $\Omega_{\odot}=30.24\pm0.12$ km s$^{-1}$ kpc$^{-1}$. We
also applied the axisymmetric model to mock data from an N-body/hydrodynamic
simulation of a Milky Way-like galaxy with a bar and spiral arms. We find that
our axisymmetric model fit to the young stars recovers the local centrifugal
speed reasonably well, even in the face of significant non-axisymmetry.
However, the local centrifugal speed determined from our Cepheid sample could
suffer from systematic uncertainty as large as 6 km s$^{-1}$.
