The excitation of emission lines in the spectra of early-type stars

Abstract

In a plane-parallel atmosphere, in radiative and local thermodynamic equilibrium, emission lines cannot normally arise. The presence of emission lines requires either a combination of thermal re-emission in the line with scattering in the continuum ( Schuster-Underhill mechanism); or fluorescence; or scattering in a moving envelope. The various basic mechanisms for the production of emission lines are discussed: Of stars show the effects of selective fluorescence, while in WR, P Cygni and Be stars Zanstra's primary fluorescence mechanism appears to be operative. Rottenberg's effect of the scattering of stellar line radiation gives an elegant explanation of the P Cygni and Be profiles, but needs to be supplemented by a consideration of fluorescence effects before it can be applied to the higher members of a series. The analysis of line intensities in WR and P Cygni stars according to Beals' Hypothesis is discussed in some detail, with special reference to the statistical equilibrium of hydrogen and helium in envelopes. Finally the transfer of radiation in envelopes is discussed, with special reference to Sobolev's theory, in which self-absorption in a moving envelope is taken into account in a very elegant manner. It is concluded that Sobolev's theory offers much new insight, but is not universally applicable; for example, the measured ratio of Paschen to Balmer lines in the spectrum of P Cygni suggests that there is little self-absorption present in the higher members of the series.