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

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 TLUSTY non-local thermodynamic equilibrium models and assumed a simple homogeneous magnetic field. The derived atmospheric parameters Teff = 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 (vrot 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.

The population of hot subdwarf stars studied with Gaia

“…In addition, several recent studies reported the discovery of individual subdwarfs (Hillwig et al 2017;Kupfer et al 2017aKupfer et al ,b, 2020Kupfer et al , 2022Holdsworth et al 2017;Ratzloff et al 2019Ratzloff et al , 2020Kilkenny et al 2019;Bell et al 2019;Hogg et al 2020;Silvotti et al 2021;Pelisoli et al 2021;Vos et al 2021;Dorsch et al 2022;Silvotti et al 2022;Werner et al 2022a,b), all of which are included in our compilation.…”

A massive helium star with a sufficiently strong magnetic field to form a magnetar

“…Stellar mergers have been proposed as a potential origin of magnetic stars ( 40 – 43 ). Strong magnetic fields have been identified in low-mass helium stars (classified as OB-type subdwarfs) that have been suggested to originate from merger events between two white dwarfs ( 13 , 14 ). However, to produce a ≈2M⊙ helium star through a merger of white dwarfs, the merger would need to have involved massive CO or ONe white dwarfs, which are rare.…”

“…However, corresponding magnetic fields have not been detected in evolved massive stars ( 12 ). Strongly magnetic low-mass helium stars have been observed ( 13 – 15 ), but not massive magnetic helium stars exceeding the Chandrasekhar limit.…”

He-rich Hot Subdwarf Stars Observed in Gaia DR3 and LAMOST DR7: Carbon and Nitrogen Abundances and Kinematics

“…Recently, Werner et al (2022) discovered a new class of He-rich sdO stars significantly enriched in C and O (CO-sdO), possibly explained by the merger of a HeWD and a less massive CO-core WD (Miller Bertolami et al 2022). Additionally, Dorsch et al (2022) reported the discovery of a highly magnetic He-sdO star resulting from a double-degenerate binary merger. Subsequently, Pelisoli et al (2022) identified three new magnetic Herich hot subdwarf stars but was unable to precisely determine the type of merger or the surface C abundance.…”