Scattering of Stars by Transient Spiral Waves
  Transient spiral waves of moderate amplitude cause substantial changes to the angular momenta of many stars in a galaxy disk. Stars near to corotation are affected most strongly: for a wave of ~20% overdensity, the rms change for particles near to corotation is ~18% of their initial angular momenta. Yet these changes produce no increase in random motion near corotation, while the same wave causes mild heating near its Lindblad resonances. The direction of radial migration depends both on initial location relative to corotation and on azimuth. Streamlines of cold gas become very significantly distorted before they cross, suggesting that gas is also mixed radially by this mechanism. Such continuous churning of the disk has profound implications for abundance gradients in galaxies. Furthermore, it is quite possible that the Sun has migrated significantly from its radius of birth, consistent with the claims of Wielen et al. 
J.A. Sellwood and Miguel Preto (Rutgers U)