Corotation resonance and the radial distribution of oxygen in a spiral galaxy

Abstract

A theory is proposed for the formation of the bimodal radial distribution of oxygen in the Galaxy, which displays a steep abundance gradient in the inner region and becomes flat in the middle and outer regions. The formation of this structure is associated with the following effects. First, type II supernovae, which are sources of oxygen, are concentrated in the spiral arms. Second, the enrichment rate is proportional to the rate at which a chemical element enters the spiral arm, which has the form of a density wave. This implies that the rate of oxygen production is substantially suppressed in most of the corotation circle due to the low relative velocity of the Galactic material with respect to the spiral arms and the low rate at which the arms are crossed by the Galactic material. Third, the corotation radius is near the Sun. The net result of these effects combined with turbulent diffusion is the development of the structure discussed in the paper.