Spectral analysis of the extreme helium star LSS3184

LSS3184 is a hydrogen-deficient, early B-type giant, recently found to pulsate with a period of 2.5 hours. Its photospheric parameters have been derived from optical high-resolution spectra by the method of fine analysis. The principal results are T_eff=23300 K, log g=3.35, {n_H}/{n_He}simle0.00015, {n_C}/{n_He}=0.003, {n_N}/{n_He}=0.0005, and {n_O}/{n_He}=0.0003. Hydrogen is extremely deficient. The effective temperature is consistent with broad-band visual and ultraviolet spectrophotometry and an extinction E_B-V ~ 0.27. Its previous evolution is reflected in the chemistry of the atmosphere, which contains enriched nitrogen from CNO-cycle hydrogen burning, and carbon from 3alpha helium burning. Thus LSS3184 is a true extreme helium star with a composition similar to \bd{-9}{4395}. With T_eff, log g, and pulsation properties very similar to the C-poor and N-rich helium star V652Her, evolutionary mechanisms which can result in very different surface compositions for the two stars must be examined. Based on observations obtained at the European Southern Observatory, La Silla, Chile, and with the IUE satellite retrieved from the IUE archive at the World Data Centre, Rutherford Appleton Laboratory, UK