Observations and analyses of the S VI λ1975, λ1993A lines in the IUE ultraviolet spectra of WN stars.

Abstract

From a detailed study of high resolution IUE ultraviolet spectra we identify strong emission lines near ~λ1990A as the S VI (4p^2^P^0^-4d^2^D) transitions at λ1975.21A, λ1992.56, 1993.24A in HD 187282 (WN4), HD 50896 (WN5), HD 191765 (WN6) and HD 192163 (WN6). These S VI emissions may also be weakly present in HD 151932 (WN7). Additional S VI (4s ^2^S-4p ^2^P^0^) λ2587, λ2618 emission features are also evident in the WNE spectra. Theoretical S VI line intensities are calculated using empirical nLTE model atmosphere codes adopting both spherically-symmetric, homogeneous and clumped wind structures. The observed S VI line strengths compared to the model predictions yield an estimated abundance ratio of S/He~[2-3]x10^-4^ by number. Within the observational and modelling uncertainties, this value is consistent with an intial near-solar abundance ratio, slightly modified by helium enhancement via CNO-burning, although a S-overabundance of x2-3 cannot be ruled out.