Contributions to the theory of spiral structure. II. The density wave theory and the very hot interstellar gas component

Starting from the argument that the density wave theory of spiral structure is incompatible with the existence of a stationary very hot extended component of the interstellar gas, we develop a cyclic model, in which an interstellar gas of a temperature around 10⁴ K is heated to roughly 10⁶ K by type II supernovae resulting from the enhanced star formation triggered by the density wave, and cools down again to 10⁴ K after the supernova activity subsides to be ready for shock compression in the next sweep of the density wave pattern. The time scales for heating by supernovae and subsequent cooling turn out to be consistent with current density wave theory. The scenario proposed is corroborated by recent results on cosmic ray composition and -ray observations.