The abundance ratio of helium to hydrogen in the Orion Nebula.

Abstract

To derive the abundance ratio of helium to hydrogen in the Orion Nebula, the strengths of the Hei lines X 4027, 4471, and 5876 were measured relative to the neighboring Balmer lines at two positions in the nebula close to the Trapezium stars. In order to relate the line intensities to the relative numbers of atoms, the theory of statistical equilibrium for the triplet levels of helium in a dilute radiation field was developed. The depopulation of the 2 S level of helium was found to take place by transitions to the ground state via two-photon emissions and superelastic collisions. The population of all triplet levels was thus determined only by the effective temperature P0. The effective value of P0 as determined by the ratio of the EOiiiJ lines X4363 and N1 N2 was io,ooo0K; the value of the electron density indicated by the ratio of the EOiiJ lines X3726 and 3729 was 3800 cm-3. The abundance ratio finally determined is He/H = 0.13 by number, or He/(H He) = 0.35 by mass. The various line ratios used gave values within twenty per cent of this average. Caloforn ia Institute of Technology, Pasadena, Calof.