SPECTRAL-ANALYSIS OF LSE-78 - AN EXTREME HELIUM STAR SIMILAR TO BD-9-DEGREES-4395 AND DY CENTAURI

LSE 78 is a hydrogen-deficient early-B supergiant, spectroscopically s imilar to the pulsating extreme helium star BD 9-degrees-4395 and the hot R CrB star DY Cen. The photospheric parameters of LSE 78 have been derived from optical high-resolution spectra by the method of fine an alysis. The principal results are T(eff) = 18 000 K, log g = 2.00, n(H )/n(He) < 0.0001, n(C)/n(He) = 0.01; hydrogen is extremely deficient. The effective temperature is consistent with broadband visual and ultr aviolet spectrophotometry and an extinction E(B-V) approximately 0.25, subject to peculiar extinction in the UV (lambda < 2000 angstrom). As suming a He-shell burning model, a mass approximately 0.8 M. may be es timated for LSE 78. Its previous evolution is reflected in the chemist ry of the atmosphere, which contains enriched nitrogen from CNO-cycle hydrogen burning, carbon from 3alpha helium burning, and oxygen from a lpha-capture reactions. The latter makes LSE 78 the most oxygen-rich e xtreme helium star analysed so far, with n(C)/n(O) approximately 3. Th is low ratio indicates that the atmospheric carbon and oxygen are the products of near complete helium burning. The iron abundance is sim0.2 times solar. Weak and variable emission lines due to He I and C II ar e detected. but are never as strong as those detected in BD-9-degrees- 4395 or DY Cen. By analogy, they provide evidence for a recombination shell, or extended atmosphere, with a temperature slightly below T(eff ), around LSE 78.