Cosmic rays, spiral structure and molecular clouds in the Galaxy.

In the light of recent work concerning the structure and content of the galactic disk, the interpretation of the high-energy galactic gamma-ray emission observed by the SAS-2 and COS-B satellites is reexamined. In particular, it is shown that the contribution of point sources, while difficult to determine, is probably nowhere dominant; that the degree of penetration of cosmic rays in molecular clouds cannot yet be known; and that all of the data available at present point to the existence of a gradient in galactic cosmic-ray density and a correlation between gamma-ray emission and spiral structure. A revised version of the type of model discussed by Paul et al., (1976) is presented which is consistent with cosmic rays seeing only half the matter in molecular clouds and with the existence of a variation with galactocentric distance of the abundance of C-13 relative to H.