EC 19529-4430: SALT identifies the most carbon- and metal-poor extreme helium star
Abstract
EC 19529-4430 was identified as a helium-rich star in the Edinburgh-Cape (EC) Survey of faint-blue objects and subsequently resolved as a metal-poor extreme helium (EHe) star in the Southern African Large Telescope (SALT) survey of chemically peculiar hot subdwarfs. This paper presents a fine analysis of the SALT high-resolution spectrum. EC 19529-4430 has $T_{\rm eff} = 20\, 700 \pm 250$ K, $\log g /{\rm cm\, s^{-2}} = 3.49\pm 0.03$, 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.
- Publication:
-
Monthly Notices of the Royal Astronomical Society
- Pub Date:
- May 2024
- DOI:
- 10.1093/mnras/stae961
- arXiv:
- arXiv:2404.03972
- Bibcode:
- 2024MNRAS.530.1666J
- Keywords:
-
- stars: abundances;
- stars: chemically peculiar;
- stars: fundamental parameters;
- stars: individual (EC 19529-4430);
- Astrophysics - Solar and Stellar Astrophysics;
- Astrophysics - Astrophysics of Galaxies
- E-Print:
- 13 pages, 11 figures. MNRAS in press