Jack A. Baldwin

TL;DR: In this paper, the merits of various emission-line intensity ratios for classifying the spectra of extragalactic objects were investigated and it was shown empirically that several combinations of easily-measured lines can be used to separate objects into one of four categories according to the principal excitation mechanism: normal H II regions, planetary nebulae, objects photoionized by a power-law continuum, and objects excited by shock-wave heating.

TL;DR: In this article, a long-slit spectrophotometric observations were obtained to redetermine the helium abundance of the Orion Nebula, and the ionic ratio He(+)/H(+) was found to remain nearly constant at 0.088 {plus minus} 0.006.

TL;DR: In this article, a large grid of photoionization models covering the widest possible range of cloud gas density and distance from the central continuum source was computed, and it was shown that for each line, only a narrow range of density and distances from the source results in maximum reprocessing efficiency, corresponding to locally optimally emitting clouds.

TL;DR: In this article, M.M.H. and M.J.C. acknowledge support provided for this work by the National Science Foundation through grant number======GF-1002-96 from the Association of Universities for Research in Astronomy, Inc., under NSF Cooperative Agreement No. AST-8947990 and from Fundacion Andes under======Project C-12984.