A differential analysis of UV photospheric lines of OBN and OBC stars.

Abstract

The paper presents measurements of the equivalent widths of several photospheric C, N, O, He, Si and Fe lines in the ultra-violet spectra of 55 OBN, OBC and normal supergiants and main-sequence stars. Seventy-two short-wavelength, high-resolution spectrograms obtained with IUE were used. The differences in equivalent widths of OBN and OBC stars relative to normal stars are interpreted as abundance differences and yield upper and lower limits to respectively under- and overabundances of the elements C, N and O. The effects of anomalous strength of CNO lines appear to be less pronounced in the UV than in the visual. The main reason for this is the saturation of the lines investigated. Different scenarios to bring products of the CNO-cycle to the stellar surface are discussed. The OBN stars can be explained in terms of abundance differences due to evolutionary effects as well as initial differences. The OBC stars may be explained by differences in atmospheric structure or initial abundances.