Galactic Rotation Based on OB Stars from the Gaia DR2 Catalogue
  We have studied a sample containing about 6000 OB stars with proper motions
and trigonometric parallaxes from the Gaia DR2 catalogue. The following
parameters of the angular velocity of Galactic rotation have been found:
$\Omega_0=29.70\pm0.11$ km s$^{-1}$ kpc$^{-1}$, $\Omega'_0=-4.035\pm0.031$ km
s$^{-1}$ kpc$^{-2}$, and $\Omega''_0= 0.620\pm0.014$ km s$^{-1}$ kpc$^{-3}$.
The circular rotation velocity of the solar neighborhood around the Galactic
center is $V_0=238\pm5$ km s$^{-1}$ for the adopted Galactocentric distance of
the Sun $R_0=8.0\pm0.15$ kpc. The amplitudes of the tangential and radial
velocity perturbations produced by the spiral density wave are
$f_\theta=4.4\pm1.4$ km s$^{-1}$ and $f_R=5.1\pm1.2$ km s$^{-1}$, respectively;
the perturbation wavelengths are $\lambda_\theta=1.9\pm0.5$ kpc and
$\lambda_R=2.1\pm0.5$ kpc for the adopted four-armed spiral pattern. The Sun's
phase in the spiral density wave is $\chi_\odot=-178^\circ\pm12^\circ$.
