SPECTRAL-ANALYSIS OF DY CENTAURI, A HOT R-CORONAE-BOREALIS STAR WITH AN UNUSUALLY HIGH HYDROGEN CONTENT

DY Cen is a hydrogen-deficient early-B supergiant, spectroscopically similar to the pulsating extreme helium star BD-9-degrees-4395. It has reputedly shown R CrB type variations and has therefore also been likened to the hot R CrB stars MV Sgr and V348 Sgr. The photospheric parameters of DY Cen have been derived from optical high-resolution spectra by the method of fine analysis, using up-to-date atomic data and new line-blanketed model atmospheres. The principal results are T(eff) = 19 500 K, log g = 2.15, n(H)/n(He) = 0.1, n(C)/n(He) = 0.01. Broad-band visual and ultraviolet spectrophotometry also yield T(eff) = 19 500 K, with E(B-V) = 0.5. The hydrogen abundance indicates that EHe stars show a much larger range of hydrogen-deficiency than previously supposed. Assuming a He-shell model, a mass approximately 0.8 M. may be estimated for DY Cen. DY Cen is a metal poor EHe star; the composition supports evidence for an anticorrelation between hydrogen and iron abundances in EHe stars. Its previous evolution is reflected in the chemistry of the atmosphere, which contains enriched nitrogen from CNO-cycle hydrogen burning, carbon from 3alpha helium burning, and oxygen and possibly neon from alpha-capture reactions. A weak emission-line spectrum principally contains lines due to H I, He I, C II and Si II. By analogy with BD-9-degrees-4395, it is probably due to recombination in a shell, or extended atmosphere, with a temperature slightly below T(eff).