Two-component outer ring and the Galactic spiral structure
  Model of the Galaxy with the ring R1R2' can explain some large-scale morphological features of the Galactic spiral structure. The Carina-Sagittarius arm can consist of two ascending segments of the outer rings R1 and R2 which almost touch each other near the Carina region. The Perseus and Crux arms can be partially identified with the descending segments of the ring R2. Model of the two-component outer ring can also explain the existence of some maxima in diagrams (l, VLSR) which are supposed to correspond to the directions tangential to the spiral arms. On the basis of numerical simulations we propose two sketches of the ring structure of the Galaxy which include the bar, two outer rings, the inner ring, and the nuclear gas condensation, that may be a nuclear ring. Both sketches can explain the position of the Carina-Sagittarius arm with respect to the Sun. 
A. M. Mel'nik and P. Rautiainen
astro-ph.GA