Discovery of a highly magnetic He-sdO star from a double-degenerate binary merger
M. Dorsch, Nicole Reindl, Ingrid Pelisoli, U. Heber, S. Geier, Alina Istrate, Stephen Justham
Abstract
Helium-rich hot subdwarf stars of spectral type O (He-sdO) are considered prime candidates for stellar merger remnants. Such events should lead to the generation of strong magnetic fields. However, no magnetic He-sdO has yet been unambiguously discovered despite the high magnetic rate (20%) among white dwarf stars, the progeny of hot subdwarfs. Here we present the discovery of a strong magnetic field ( B = 353 ± 10 kG) from Zeeman-split hydrogen, helium, and metal lines in the optical X-shooter spectrum of an He-sdO and present the first spectroscopic analysis of any magnetic hot subdwarf. For this we used line-blanketed T LUSTY non-local thermodynamic equilibrium models and assumed a simple homogeneous magnetic field. The derived atmospheric parameters T eff = 44 900 ± 1000 K and log g = 5.93 ± 0.15 are typical for He-sdO stars, while the star is less hydrogen-poor than most He-sdOs at log n (He)/ n (H) = + 0.28 ± 0.10. The star is a slow rotator ( v rot sin i < 40 km s −1 ). Its chemical composition is N-rich and C- and O-poor, and the Si and S abundances are close to solar. Combining the atmospheric parameters with Gaia parallax and photometry, the stellar radius and luminosity are found to be typical for He-sdOs and place the star on the helium main sequence in the Hertzsprung-Russell diagram. Its mass of 0.93 −0.30 +0.44 M ⊙ , although uncertain, appears to be remarkably high. The strong magnetic field along with the atmospheric parameters and metal abundances provide overwhelming evidence for the double-degenerate merger scenario.