Cepheid Kinematics and the Galactic Warp

Abstract

The space velocities of 200 long-period ($P>5$ days) classical Cepheids with known proper motions and line-of-sight velocities whose distances were estimated from the period--luminosity relation have been analyzed. The linear Ogorodnikov-Milne model has been applied, with the Galactic rotation having been excluded from the observed velocities in advance. Two significant gradients have been found in the Cepheid velocities, $\partial W/\partial Y=-2.1\pm0.7$ km s$^{-1}$ kpc$^{-1}$ and $\partial V/\partial Z= 27\pm10 $ km s$^{-1}$ kpc$^{-1}$. In such a case, the angular velocity of solid-body rotation around the Galactic $X$ axis directed to the Galactic center is $-15\pm5$ km s$^{-1}$ kpc$^{-1}$.