The Statistical Equilibrium of Hydrogen and Helium in a Radiation Field, with an Application to Interpreting Wolf-Rayet Spectra

Abstract

A 70-level model helium atom which includes singly and doubly excited states of He I and a 17-level model hydrogen atom are used to solve the equations of statistical equilibrium for helium and hydrogen in a radiation field. The plasma has a velocity gradient, and densities and temperatures typical for the mantles of Wolf-Rayet stars. The emission lines of the Wolf-Rayet stars HD 50896, HD 191765, HD 192103, and HD 192163 are analyzed, as well as those of ζ Pup (O4ef) and α Cam (O9.5 Ia). It is argued that the apparent decrement of intensity along the Pickering series of He II is an ambiguous indicator of the composition of the atmospheres of Wolf-Rayet stars. The electron temperatures in the atmospheres of Wolf-Rayet stars probably exceed 10⁵K.