The helium-rich subdwarf CPD-20°1123: a post-common-envelope binary evolving on to the extended horizontal branch

Naslim, N., Geier, S., Jeffery, C.S., Behara, N.T., Woolf, V. M., Classen, L.

Figure 5: The location of CPDòÈÒ20° 1123 on the log g–T_eff diagram, compared with normal sdB stars (Edelmann et al. 2003), helium-rich sdB stars (Ahmad & Jeffery 2003; Naslim et al. 2010), helium-rich sdO stars (Stroeer et al. 2007), LS IVòÈÒ14° 116 (Naslim et al. 2011) and JL 87 (Ahmad et al. 2007). A zero-age main sequence (Y = 0.28, Z = 0.02), a zero-age horizontal branch (M_c = 0.485M_☉, Y_e = 0.28, Z = 0.02) and a helium main sequence (Z = 0.02) are also shown.

Abstract

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 η_He < 1 per cent), whilst a minority have extremely helium rich surfaces (η_He > 90 per cent). A small number have an intermediate surface helium abundance (≈10–30 per cent), 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 a 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 CPDòÈÒ20°1123 (Albus 1) which shows it to be cool, for a hot subdwarf, with Tsub>eff ≈ 23 000 K and with a surface helium abundance ≈17 per cent 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.

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Last Revised: 2012 September 26th