Discovery of a highly magnetic He-sdO star from a double-degenerate binary merger

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