The Chemical Composition of the Wolf-Rayet Stars

Abstract

This review summarises current knowledge of the chemical composition of PopI WR stars, concentrating on work carried out in this area since the last IAU, No. 49, symposium devoted to this stellar class (Bappu & Sahade 1973). Earlier reviews of this topic are found in Gebbie & Thomas (1968). The dichotomy of the WR stars into the WN and WC sequences (Beals 1934) has generally been qualitatively interpreted as arising because of gross differences in the C and N abundances: WN stars which exhibit emission lines of predominantly He and N ions with little evidence for C, being inferred as C-poor objects, whilst WC stars, showing predominantly He and C lines and virtually no evidence for N being inferred as N-poor. In both sequences the visible spectra show little or no evidence for hydrogen. However, although the WR stars have been acknowledged as a class for over a century now, progress has been very slow in putting quantitative determinations of their physical and chemical properties on a firm basis, with the bulk of work in this area being conducted during the past decade. The chemical nature of the WR stars has always been a matter of considerable uncertainty, controversy and, quite often, passionate disagreement, arising from uncertainties in the interpretation of the, often ambiguous,observational material available, as well as from disagreements as to the reliability of the use of comparatively simple analytical models employed to date.