Armagh Observatory Preprint Series No. 306

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The chemical composition of the pulsating helium star V652 Her

Authors: C.S.Jeffery, P.W.Hill, U.Heber

Journal: 1999, A&A submitted

Abstract. We present an analysis of an optical blue spectrum of the pulsating helium star V652 Her (=BD+13 3224$) in order to determine its effective temperature, surface gravity and chemical composition. By fitting synthetic spectra to the observations we find that for our spectrum Teff =24 550+/-500 K, log g=3.68+/-0.05 (cgs) and v_t=5+/-5 km/s. The surface gravity, together with a previous measurement of the stellar radius, indicates the mass of V652 Her to be M=0.69(+0.15/-0.12)\Msun. The surface composition is characterised by abundances of nH=0.009, He=0.988, nC~0.000040, nN=0.0025 and nO=0.00010 (number fractions). These abundances represent a mixture of some hydrogen-rich material (0.2% by mass) with predominantly CNO-processed helium (99.8% by mass). The metallicity of V652 Her, represented by the N abundance as a sum of primordial C+N+O abundances, by the iron abundance, and by other metals, corresponds to a near-solar mixture, with [Fe/H]=-0.10+/-0.15. Such a metallicity supports the contention that Z-bump opacities drive pulsations in metal-rich helium stars in an instability finger that extends to low luminosities for stars with Teff~20\,000 K. There is no evidence for the products of any nuclear processes other than the CNO cycle on the stellar surface. If V652 Her was formed by the merger of two white dwarfs, its surface composition demands that they should both be helium white dwarfs. Conversely, 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 helium main-sequence, with important consequences for understanding the origin of hot subluminous stars. A small discrepancy remains between Teff and log g measured from the average blue-visual spectrum in this paper, and that measured from UV-optical spectrophotometry previously. Further work will be necessary to resolve this, and to make progress in determining the mass of V652 Her.

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