EC 19529–4430: SALT identifies the most carbon- and metal-poor extreme helium star

Abstract

Abstract EC 19529−4430 was identified as a helium-rich star in the Edinburgh-Cape Survey of faint-blue objects and subsequently resolved as a metal-poor extreme helium (EHe) star in the SALT survey of chemically-peculiar hot subdwarfs. This paper presents a fine analysis of the SALT high-resolution spectrum. EC 19529−4430 has Teff = 20 700 ± 250 K, log g/cm s−2 = 3.49 ± 0.03, and an overall metallicity some 1.3 dex below solar; surface hydrogen is $\approx 0.5~{{\ \rm per\ cent}}$ by number. The surface CNO ratio 1:100:8 implies that the surface consists principally of CNO-processed helium and makes EC 19529−4430 the coolest known carbon-poor and nitrogen-rich EHe star. Metal-rich analogues include V652 Her and GALEX J184559.8−413827. Kinematically, its retrograde orbit indicates membership of the galactic halo. No pulsations were detected in TESS photometry and there is no evidence for a binary companion. EC 19529−4430 most likely formed from the merging of two helium white dwarfs, which themselves formed as a binary system some 11 Gyr ago.