Spectral analysis of the extreme helium star LSS 3184

LSS 3184 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) less than or similar to 0.00015, 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 EB-V similar to 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 3 alpha helium burning. Thus LSS 3184 is a true extreme helium star with a composition similar to BD-9 degrees 4395. With T-eff, log g, and pulsation properties very similar to the C-poor and N-rich helium star V652 Her, evolutionary mechanisms which can result in very different surface compositions for the two stars must be examined.