The chemical composition of the pulsating helium star V652 Her

We present an analysis of an optical blue spectrum of the pulsating helium star V652 Her (=BD+13 degrees 3224) in order to determine its effective tem perature, surface gravity and chemical composition. By fitting synthetic sp ectra to the observations we find that for our spectrum T-eff = 24 550 +/- 500 K, log g = 3.68 +/- 0.05 (egs) and v(t) = 5 +/- 5 km s(-1). The surface gravity, together with a previous measurement of the stellar radius, indic ates the mass of V652 Her to be nr = 0.69(-0.12)(+0.15) M.. The surface com position is characterised by abundances of n(H) = 0.009, n(He) = 0.988; n(C ) similar or equal to 0.000040, n(N) = 0.0025 and n(O) = 0.00010 (number fr actions). These abundances represent a mixture of some hydrogen-rich materi al (0.2% by mass) with predominantly CNO-processed helium (99.8% by mass). The metallicity of V652Her, represented by the N abundance as a sum of prim ordial C+N+O abundances, by the iron abundance, and by other metals, corres ponds to a near-solar mixture, with [Fe/H]= -0.10 +/- 0.15. Such a metallic ity supports the contention that Z-bump opacities drive pulsations in metal -rich helium stars in an instability finger that extends to low luminositie s for stars with T-eff similar to 20 000 K. There is no evidence for the pr oducts of any nuclear processes other than the CNO cycle on the stellar sur face. If V652 Her was formed by the merger of two white dwarfs, its surface composition demands that they should both be helium white dwarfs. Converse ly, if it is the product of single-star evolution, it is more likely to be a post-giant branch star. In either case it is probably evolving onto the h elium main-sequence, with important consequences for understanding the orig in of hot subluminous stars. A small discrepancy remains between T-eff and log g measured from the average blue-visual spectrum in this paper, and tha t measured from W-optical spectrophotometry previously. Further work will b e necessary to resolve this, and to make progress in determining the mass o f V652 Her.