Some of the peculiar features of the periodic velocity-field structure for OB associations can be explained by using the Roberts-Hausman model , in which the behavior of a system of dense clouds is considered in a perturbed potential .
The absence of statistically significant variations in the azimuthal velocity across the Carina arm , probably , results from its sharp increase behind the shock front , which is easily blurred by distance errors .
The existence of a shock wave in the spiral arms and , at the same time , the virtually free motion of OB associations in epicycles can be reconciled in the model of particle clouds with a mean free path of 0.2 \text { - - } 2 kpc .
The velocity field of OB associations exhibits two appreciable nonrandom deviations from an ideal spiral pattern : a 0.5-kpc displacement of the Cygnus- and Carina-arm fragments from one another and a weakening of the Perseus arm in quadrant III .
However , the identified fragments of the Carina , Cygnus , and Perseus arms do not belong to any of the known types of spurs . Key words : kinematics and dynamics , Milky Way Galaxy , spiral pattern , OB associations .