Chemical Composition of the Planetary Nebulæ

Abstract

THE physical conditions in the planetary nebulæ are such that atoms or ions in metastable states are not disturbed by electron impact or the absorption of radiation during their lifetimes of one minute or more. The intensity of a forbidden emission line (due to transitions from such a metastable state) is then¹ I = nhv, where n is the number of ions arriving in the metastable state per second, and hv is the energy emitted from each transition. It is possible to determine the relative abundance of ions in the nebulae from the observed² intensities of their forbidden lines (listed in cols. 5 and 7 of the Table), if, following Bowen, we assume the excitation to the metastable state to be due solely to inelastic electron collisions, that is, n is proportional to N(w)p(v,w)(E), where N{w) is the number of ions of atomic weight w per c.c, p(v,w) is the probability of electron collision and resultant excitation, and (E) is the number of electrons per c.c. with kinetic energy greater than the excitation potential, E, of the ion.