Optical and Radio Evidence of Large-Scale Peculiar Motions in the Cassiopeiaperseus Arm

Recent optical data on young open clusters, H ii regions, 0 associations, interstellar extinction, and interstellar absorption lines together with new high-resolution 21-cm line hydrogen maps of the Cas-Per spiral arm (970 < 1 < 1500, - 1?O «= b ~ 1°0) indicate that the interstellar medium in a large section of the arm is moving with peculiar radial velocities of the order of -20 to -30 km sec A distance esti- mate based upon the interstellar absorption lines, interstellar extinction, and H ii regions indicates that the gas, dust, and ionized elements are situated 1 5 to 2 5 kpc from the Sun. A new determination of Oort's constant A shows that the extremely young clusters with H ii regions form a peculiarly moving kinematic group (group F), distinct from the normally moving 0 associations and other young clusters (group N). The age of the group P objects is about 6 X 106 years, while that of the group N objects is estimated to be 10-30 X 106 years. The mass and gas-to-dust ratio of the Cas- Per arm are also calculated. A model which explains the observed peculiar motions and the age differences is an expanding ring or shell in the galactic plane This ring contains about 1O~ Mo and is expanding at 20-30 km sec1 from the direction 1 = 120~. It is approximately 1600 Pc ill diameter and is similar to rings which have been observed in the Magellanic Clouds The ring is postulated to have been caused by one super-supernova, a small number of large supernovae, or a large number of ordinary supernovae. The explosion took place less than 30 X 106 years ago, and the object producing the ring must have been a group N member; the peculiarly moving group P objects are much younger and were formed from the expanding ring. The high- and intermediate-velocity hydrogen clouds reported by the Dutch are also included in the model. They indicate that the explosion took place slightly above the plane