Kinematic evidence for the wave nature of the Carina-Sagittarius arm

Abstract

Radial-velocity and (Hipparcos) proper-motion data are used to construct the residual space velocity field for OB associations in the Carina-Sagittarius arm. The azimuthal residual velocity component for most stars in the Sgr OB1, Ser OB1, Cru OB1, Cen OB1, and Car OB2 associations has the direction that is opposite to the Galactic rotation, while in the Car OB1 association this component has the same direction as the Galactic rotation. The radial residual velocity component for Cru OB1 and Cen OB1 is directed toward the Galactic center, while for Car OB2, Car OB1, and Sgr OB1 it is close to zero. We found a residual-velocity gradient for Cru OB1, Car OB2, and Car OB1 from the inner edge of the Carina arm (V_theta = 10 km s^-1; V_R = 9 km s^-1) to its outer edge (V_theta = +4 km s^-1; VR = +3 km s^-1). We argue that the change in the residual velocities of the associations from the inner to the outer edges of the Carina arm is accompanied by the stratification of stars in age. Our results are in general agreement with the expected residual-velocity distribution due to the spiral density waves within the corotation radius.