Planetary nebulae and nitrogen enrichment in the galaxy

Abstract

The extent of nitrogen enrichment of the interstellar gas by planetary-nebula formation is evaluated, and the nitrogen and oxygen abundance gradients in the disk of the Galaxy are derived on the basis of a simple model for galactic evolution. The double-shell-source lifetime of stars of different initial mass is determined along with the maximum possible number of helium shell flashes for a star of particular mass. CNO production via the shell flashes is examined, the N-14 and O-16 yields for a generation of stars are computed, and the distributions of oxygen and nitrogen in the Galactic disk are obtained. It is found that primary production of N-14 in low-mass stars contributes nearly 50 percent to the total abundance of N-14 in the Galaxy, that the logarithmic abundance gradients in the solar neighborhood are -0.18 for nitrogen and -0.13 for oxygen, and that the gradients are sensitive to the distribution of interstellar gas.