Friday, April 20, 2012

1204.4387 (Naslim. N et al.)

The helium-rich subdwarf CPD-20 1123: a post-common envelope binary evolving onto the extended horizontal branch    [PDF]

Naslim. N, S. Geier, C. S. Jeffery, N. T. Behara, V. M. Woolf, L. Classen
Subluminous B stars come in a variety of flavours including single stars, close and wide binaries, and pulsating and non-pulsating variables. A majority have helium-poor surfaces (helium by number nHe<1%), whilst a minority have extremely helium-rich surfaces (nHe>90%). A small number have an intermediate surface helium abundance (~ 10 - 30%), accompanied by peculiar abundances of other elements. The questions posed are i) whether these abundance peculiarities are associated with radiatively-driven and time-dependent stratification of elements within the photosphere as the star evolves from an helium-enriched progenitor to become a normal helium-poor sdB star, and ii) whether these phenomena occur only in single sdB stars or are also associated with sdB stars in binaries. We present a fine analysis of the bright intermediate-helium sdB star CPD-20 1123 (Albus 1) which shows it to be cool, for a hot subdwarf, with Teff~23\,000 K and with a surface helium abundance ~17% by number. Other elements do not show extraordinary anomalies; in common with majority sdB stars, carbon and oxygen are substantially depleted, whilst nitrogen is enriched. Magnesium through sulphur appear to be depleted by ~0.5 dex, but chlorine and argon are substantially enhanced. We also present a series of radial-velocity measurements which show the star to be a close binary with an orbital period of 2.3 d, suggesting it to be a post-common-envelope system. The discovery of an intermediate helium-rich sdB star in a close binary in addition to known and apparently single exemplars supports the view that these are very young sdB stars in which radiatively-driven stratification of the photosphere is incomplete.
View original: http://arxiv.org/abs/1204.4387

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