Open Cluster Birth Analysis and Multiple Spiral Arm Sets in the Milky   Way
  The Milky Way spiral arm dynamics is studied using the birth place of open clusters. We separately study the nearby spiral arms, and find evidence for multiple spiral sets. In particular, the Sagittarius-Carina arm appears to be a superposition of two sets. The first has a pattern speed of Omega_P,Carina,1=16.5^{+1.2}_{-1.4sys}+/-1.1_{stat} (km/s)/kpc, while the second has Omega_P,Carina,2=29.8^{+0.6}_{-1.3sys}+/-1.3_{stat} (km/s)/kpc. The Perseus arm located at larger galactic radii exhibits only one clear set with Omega_P,Perseus=20.0^{+1.7}_{-1.2sys}+/-1.6_{stat} (km/s)/kpc. The Perseus and "slower" Carina arms are most probably part of an m=4 set. The Orion "armlet" appears to have a pattern speed of Omega_P,Orion=28.9 ^{+1.3}_{-1.2sys}+/-0.8_{stat} (km/s)/kpc. In other words, both the "faster" Carina arm and the Orion arm are nearly corotating with the solar system. 
Smadar Naoz, Nir J. Shaviv