Galactic masers: kinematics, spiral structure and the disk dynamic state
  We applied the currently most comprehensive version of the
statistical-parallax technique to derive kinematical parameters of the maser
sample with 136 sources. Our kinematic model comprises the overall rotation of
the Galactic disk and the spiral density-wave effects. We take into account the
variation of radial velocity dispersion with Galactocentric distance. The best
description of the velocity field is provided by the model with constant radial
and vertical velocity dispersions, $(\sigma U0, \sigma W0) \approx (9.4 \pm
0.9~, 5.9 \pm 0.8)~ km/s$. We compute flat Galactic rotation curve over the
Galactocentric distance interval from 3 to 15 kpc and find the local circular
rotation velocity to be $ V_0 \approx (235-238)$~ km/s $\pm 7$~ km/s. We also
determine the parameters of the four-armed spiral pattern (pitch angle $i
\approx (-10.4 \pm 0.3)^\circ$ and the phase of the Sun $\chi_0 \approx (125
\pm 10) ^\circ$). The radial and tangential spiral perturbations are about $f_R
\approx (-6.9 \pm 1.4)$~km/s, $f_\Theta \approx (+2.8 \pm 1.0$) ~km/s. The
kinematic data yield a solar Galactocentric distance of $R_0 \approx (8.24 \pm
0.12)~kpc$. Based on rotation curve parameters and the asymmetric drift we
Infer the exponential disk scale $H_D \approx (2.7 \pm 0.2)$ ~kpc under
assumption of marginal stability of the intermediate-age disk, and finally we
estimate the minimum local surface disk density, $\Sigma (R_0) > (26 \pm 3) ~
M_\odot pc^{-2}$.
