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Infrared | (1. 0-3. 4 m) | recombination lines of six WC stars are analysed following the method of Hummer, Barlow & Storey. Improved spectral resolution and increased wavelength coverage give more consistent abundance estimates, for a larger number of recombination lines, compared to previous studies. Some low-energy-level transitions are found to be optically thick or formed through other processes. There is a clear decrease of derived ionic abundances with wavelength, attributed to an increase in continuous free–free opacity. Based on this observation, it is shown that the ionization stratification of the wind can be estimated, and that C/He abundance ratios can be reliably determined by defining a zone in the stellar wind, the same for all ions, in which ionic abundances are derived, thus overcoming previous difficulties inherent in the method. The derived C/He ratios range from 0. 07 (WC9) to 0. 41 (WC5), confirming a trend with we subtype (ionization degree) for our small sample, with however the exception of the anomalous star WR 146, which has a low C/He ratio and a very high ionization degree. The C + + + +/He+ + ratio is found to provide a good estimate of the C/He ratio.
Eenens et al. (Sun,) studied this question.