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_c - defined as the rotation speed required to balance the local radial gravitational force - at the Sun's location to be V_c(R₀) = 236 ± 3 km s^-1 and the Sun's azimuthal and radial peculiar motions to be V_⊙ = 12.4 ± 0.7 km s^-1 and U_⊙ = 7.7 ± 0.9 km s^-1, respectively. These results are obtained with strong priors on the solar radius, R₀ = 8.2 ± 0.1 kpc, and the Sun's angular rotation velocity, Ω_⊙ = 30.24 ± 0.12 km s^-1 kpc^-1. We also applied the axisymmetric model to mock data from a 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.