Streaming motions of molecular clouds, ionized hydrogen, and OB stars in the Cygnus arm

Abstract

The radial velocity fields of molecular clouds, OB stars, and ionized hydrogen in the Cygnus arm (l ∼ 72°–8°) are analyzed. A gradientΔVLSR/Δlin the mean line-of-sight velocities of molecular clouds and ionized hydrogen due to differential Galactic rotation is detected, and two groups of physically and genetically associated objects moving with different line-of-sight velocities are identified. One of the two molecular-cloud complexes (l∼77.3°–80°) is located within 1 kpc of the Sun, closer to the inner edge of the arm, whereas the other complex (l∼78.5°–85°) lies 1–1.5 kpc from the Sun and is farther from the inner edge of the arm. The residual azimuthal velocities of the objects in both groups are analyzed. The residual azimuthal velocities of the first molecular-cloud complex are directed opposite to the Galactic rotation (VΘ ∼ −7 km/s), while those of the second complex are near zero or in the direction of Galactic rotation, independent of the distance to the complex (VΘ ≥ 1 km/s). Like the molecular clouds, stars of the Cygnus arm form two kinematic groups with similar azimuthal velocities. On the whole, the mean azimuthal velocities VΘ for the ionized hydrogen averaged over large areas agree with the velocities of either the first or second molecular-cloud complex. In terms of density-wave theory, the observed differences between the magnitudes and directions of the azimuthal velocities of the kinematic groups considered could be due to their different locations within the arm.