Spectrophotometry of planetary nebulae. II - Chemical abundances

Abstract

The relative abundances of H, He, Ne, O, and, less reliably, N and S have been determined for 37 planetary nebulae. Helium abundances are found to be affected by incomplete ionization of helium, an effect that is strongly correlated with the abundance of S(+). The calculations of Cox and Daltabuit (1971) are found greatly to overestimate collisional effects in helium. Helium abundances in planetaries are also apparently affected by enrichment of nebular material by CNO-processed material. There is some inconclusive evidence for a galactic radial abundance gradient of helium, nitrogen, and possibly oxygen and neon, but this is found only among the small subgroup of planetaries with low velocities and small distances above the galactic plane. The most important result is that planetaries of widely varying kinematical properties have nearly identical (approximately solar) abundances. Apparently most planetaries belong to a metal-rich population, or oxygen and neon abundances are not representative of heavy-element abundances, perhaps because oxygen and neon were synthesized at a faster rate than heavier elements in the early history of the Galaxy.