The density-wave nature of the Cygnus-Orion arm

Details The density-wave nature of the Cygnus-Orion arm The density-wave nature of the Cygnus-Orion arm

Mar 1999

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999astl...25..156s&link_type=abstract

Astronomy Letters, Volume 25, Issue 3, March 1999, pp.156-168

Astronomy and Astrophysics

Astronomy

9

Scientific paper

Radial velocities and Hipparcos proper motions are used to construct the residual space velocity field for the OB associations in the Cygnus-Orion arm. We detected systematic motions typical of density-wave arms in Cyg OB1, OB3, OB7, OB8, OB9, Sco OB2, Cep OB2 and OB3 and those typical of the interarm space within the corotation radius in Per OB2, Ori OB1, Mon OB1 and Coll 121. The radial residual velocity component of the associations in the Cygnus arm is directed toward the Galactic center. Its magnitude decreases in the arm cross section with Galactocentric distance from 10-23 km/s in Cyg OB1, OB3, OB8 and OB9 to 28 km/s in Cyg OB7, Cep OB2 and OB3. The azimuthal residual velocity component also depends on the association position 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) to that coinciding with the direction of Galactic rotation in Cyg OB7, Cep OB2 and OB3 (V_theta = 0 ... +7 km/s). In Per OB2, Ori OB1, Mon OB1, and Coll 121 located in the interarm space, the radial velocity is directed away from the Galactic center, while the azimuthal velocity (except for Coll 121) is in the direction of Galactic rotation. The two velocity components range from 4 to 12 km/s. We conclude that the Cygnus-Orion arm lies within the corotation radius. We show that variations in the parameters of the rotation curve and in the distance scale for the associations over a broad range do not result in qualitative changes in the residual velocity field of the Cygnus-Orion and Sagittarius-Carina arms and of the adjacent interarm space. We give arguments against the interpretation of stellar residual proper motions in the Cygnus associations in terms of the supershell expansion under the action of stellar wind from Cyg OB2. We detected a concentration of Cepheids toward the outer edge of the Cygnus arm and an approximately two- fold increase of their density in the arm within 1 to 2 kpc of the Sun compared to the mean density of Cepheids at the same distances.

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