Discovery and analysis of three magnetic hot subdwarf stars: evidence for merger-induced magnetic fields

TL;DR

This study reports the discovery of three magnetic hot subdwarf stars, providing evidence that their magnetic fields are likely generated by stellar mergers, which has implications for understanding magnetic field origins in stellar evolution.

Contribution

The paper presents the first identification of multiple magnetic hot subdwarfs, supporting the merger hypothesis as a source of magnetic fields in these stars and their evolutionary descendants.

Findings

Discovered three new magnetic hot subdwarfs with 300-500 kG fields.

All three stars are helium-rich O-type stars (He-sdOs).

Evidence suggests these stars formed via mergers, explaining their magnetic fields.

Abstract

Magnetic fields can play an important role in stellar evolution. Among white dwarfs, the most common stellar remnant, the fraction of magnetic systems is more than 20 per cent. The origin of magnetic fields in white dwarfs, which show strengths ranging from 40 kG to hundreds of MG, is still a topic of debate. In contrast, only one magnetic hot subdwarf star has been identified out of thousands of known systems. Hot subdwarfs are formed from binary interaction, a process often associated with the generation of magnetic fields, and will evolve to become white dwarfs, which makes the lack of detected magnetic hot subdwarfs a puzzling phenomenon. Here we report the discovery of three new magnetic hot subdwarfs with field strengths in the range 300-500 kG. Like the only previously known system, they are all helium-rich O-type stars (He-sdOs). We analysed multiple archival spectra of the three systems and derived their stellar properties. We find that they all lack radial velocity variability, suggesting formation via a merger channel. However, we derive higher than typical hydrogen abundances for their spectral type, which are in disagreement with current model predictions. Our findings suggest a lower limit to the magnetic fraction of hot subdwarfs of 0.147 (+0.143/-0.047) per cent, and provide evidence for merger-induced magnetic fields which could explain white dwarfs with field strengths of 50-150 MG, assuming magnetic flux conservation.