The density-wave nature of the Cygnus-Orion arm

Radial velocities and Hipparcos proper motions are used to construct the re sidual space velocity held for the OB associations in the Cygnus-Orion arm. We detected systematic motions typical of density-wave arms in Cyg OB1, OB 3, OB7, OB8, OB9, Sco OB2, Cep OB2 and OB3 and those typical of the interar m space within the corotation radius in Per OB2, Ori OB1, Mon OB1 and Coll 121. The radial residual velocity component of the associations Tn the Cygn us arm is directed toward the Galactic center. Its magnitude decreases in t he arm cross section with Galactocentric distance from 10-23 km s(-1) in Cy g OB1, OB3, OB8 and OB9 to 2-8 km s(-1) in Cyg OB7, Cep OB2 and OB3. The az imuthal residual velocity component also depends on the association positio n in the arm cross section; it changes direction from that opposite to the Galactic rotation in Cyg OB1, OB3, OB8, and OB9 (V-theta = -16... -3 km s(- 1)) to that coinciding with the direction of Galactic rotation in Cyg OB7, Cep OB2 and OB3 (V-theta = 0-+7 km s(-1)). In Per OB2, Ori OB1, Mon OB1, an d Cell 121 located in the interarm space, the radial velocity is directed a way from the Galactic center, while the azimuthal velocity (except for Coil 121) is in the direction of Galactic rotation. The two velocity components range from 4 to 12 km s(-1). We conclude that the Cygnus-Orion arm lies wi thin the corotation radius. We show that variations in the parameters of th e rotation curve and in the distance scale for the associations over a broa d range do not result in qualitative changes in the residual velocity field of the Cygnus-Orion and Sagittarius-Carina arms and of the adjacent intera rm space. We give arguments against the interpretation of stellar residual proper motions in the; Cygnus associations in terms of the supershell expan sion under the action of stellar wind from Cyg OB2. We detected a concentra tion of Cepheids toward the outer edge of the Cygnus arm and an approximate ly twofold increase of their density in the arm within 1 to 2 kpc of the Su n compared to the mean density of Cepheids at the same distances.