On the absorption continuum of the negative oxygen ion

Abstract

A new determination of the electron affinity of oxygen (3.07 e.v.) suggests that the ionization continuum of 0^- in the photographic ultraviolet region becomes so strong in late K and M stars that its contribution to the monochromatic absorption coefficient cannot be neglected. The influence of 0^- on the Rosseland mean is insignificant. A discontinuity in the spectra of M giants, first observed by Lindblad, is tentatively identified with the absorption edge of 0^- near λ4050. The theoretical strength of the Lindblad discontinuity depends on the abundance ratio of atomic oxygen to atomic hydrogen, which in M stars varies with the pressure-sensitive shift of the dissociation equilibrium of the several hydrides and oxides. The result is a positive absolute magnitude effect for the Lindblad discontinuity. The differential of free-oxygen abundance implied by Lindblad's spectrophotometric observations agrees as to order of magnitude with that predicted by Russell from the dissociation equilibrium in giant and dwarf stars of the oxygen branch.