Of-WN evolution: spectral types and effective temperatures

Because of the growing evidence that some Of stars are evolving into WN stars, the author reexamines the classification of WN stars. There are four schemes for classifying WN stars, those of the IAU (1938), Hiltner and Schild (1966), Smith (1968), and Walborn (1974). All of these schemes involve estimating by eye the ratio of the nitrogen line strengths. Walborn's scheme is the most useful because he avoids problems that may arise from the use of helium lines. In other respects Walborn's scheme gives WN classifications similar to those found by Smith. Equivalent widths hav been measured of lines of N III lambdalambda4640-4642, N IV lambda4058, N V lambda4604 for 22 Of stars. These Of stars were then given WN subclasses according to the criteria of Walborn (1974). The equivalent widths of the absorption lines of He I lambda 4471 and He II lambda 4541 were measured in WN transition stars: HD 93162, HD93131, HD 92740. (Auth.)