The riddle of the Wolf-Rayet stars

Abstract

The history of Wolf-Ray et stars since their discovery is noted and a brief review is given of the major studies of these rare emission-line stars. One goal of the present paper is to determine what the spectra of Wolf-Rayet stars imply concerning the physical state of the atmospheres of WolfRayet stars. A second goal is to determine what the relationship is, if any, between a particular Wolf-Rayet subtype and the evolutionary state of a massive star. Because some massive Population I stars and some low-mass evolved stars show Wolf-Ray et-type spectra, the cause of the phenomenon is not associated with only one stage of evolution.The characteristic features of Wolf-Rayet spectra are reviewed and the typical masses, radii, effective temperatures, and rates of mass loss of Population I Wolf-Rayet stars are noted. The use of modeling as a tool for determining the physical state of a Wolf-Rayet atmosphere is reviewed and evaluations are given of the analyses which have been published of Wolf-Rayet spectra. It is noted that it is essential to identify a model atmosphere with the atmosphere of a star by predicting spectral features which agree with several details in the observed spectrum. This is because possessing a particular type of emission-line spectrum is the characteristic which defines a WolfRayet star.It is concluded that Wolf-Rayet stars appear to have effective temperatures in the range from 25,000 K to 30,000 K, normal (solar) composition, a density of the order of 10 10 cm 3 in the emission-line forming regions, and an electron temperature in the range from 50,000 K to 10 5 K in the case of WC stars, but greater than 10 5 K in WN stars. The mantles of Wolf-Rayet stars are very extensive, at least 10 14 cm in radius. This picture is self-consistent; it is yet to be proved that it is unique.A brief discussion is given of possible causes of the physical state of the mantle of a Wolf-Rayet star. It is pointed out that the electrodynamic coupling of mechanical motion in a disk supported by magnetic lines of force to the plasma of the stellar mantle is a promising source for the nonradiative energy and momentum which must be deposited in the mantle of the star in order to generate a Wolf-Rayet-type spectrum. At present any postulated correlation between Wolf-Rayet subtype and stage of evolution is entirely speculative and what exists in the literature is not supported by sound spectrum analysis.