Galactic masers: Kinematics, spiral structure and the disk dynamic state

Abstract

We applied the currently most comprehensive version of the statistical-parallax technique to derive the 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, and the variation of radial velocity dispersion with Galactocentric distance. The best description of the observed velocity field is provided by the model with constant radial and vertical velocity dispersions, (σU0, σW0) ≈ (9.4 ± 0.9, 5.9 ± 0.8) kms⁻¹. The resulting Galactic rotation curve proved to be flat over the Galactocentric distance interval from 3 to 15 kpc and we find the local circular rotation velocity to be V₀ ≈ (235 − 238) ± 7 km s⁻¹. We determine the parameters of the four-armed spiral pattern (pitch angle i ≈ −10. ◦ 4 ± 0.3◦ and the phase of the Sun χ0 ≈ 125◦ ± 10◦). The amplitudes of radial and tangential spiral perturbations are about f_R ≈ −6.9 ± 1.4 km s⁻¹, f_Θ ≈ +2.8 ± 1.0 km s⁻¹. The kinematic data yield a solar Galactocentric distance of R₀ ≈ 8.24 ± 0.12 kpc. Based on rotation curve parameters and the asymmetric drift we infer the exponential disk scale H_D ≈ 2.7 ± 0.2 kpc assuming marginal stability of the intermediate-age disk, and also estimate the minimum local surface disk density, Σ(R₀) > 26 ± 3M_⊙ pc⁻².