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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.
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Magnetic Field Measurements of Low-mass Stars from High-resolution Near-infrared IGRINS Spectra
Abstract We present average magnetic field measurements derived from high-resolution near-infrared IGRINS spectra of a carefully selected sample of 28 M dwarfs. All 28 have reported magnetic field strengths in the literature. The main goal of this work is to investigate the accuracy, precision, and limitations of magnetic field measurements from IGRINS spectra. This investigation is critical to validating the robustness of our methods before we apply them to over 500 IGRINS-observed M dwarfs in the next paper of the series. We used the Zeeman broadening and Zeeman intensification methods to measure average magnetic fields. Our measurements are all consistent with the previous measurements to within ±1 kG, with an average offset of −0.17 kG for the broadening method and +0.19 kG for the intensification method. We find that the detection limit of IGRINS is ∼0.9 kG with the Zeeman broadening method, in accordance with the instrumental broadening limit of the spectrograph. With the Zeeman intensification method, we are able to detect down to ∼0.7 kG with a signal-to-noise ratio of 150 or greater. We find an advantage of using the intensification method over the broadening method, which is the ability to reliably measure the magnetic field strengths of stars that are cooler than 3100 K where the spectrum becomes dominated by molecular lines. Therefore, the intensification method is crucial to study stellar magnetism of late-M and brown dwarfs.
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- Award ID(s):
- 1908892
- PAR ID:
- 10420296
- Publisher / Repository:
- DOI PREFIX: 10.3847
- Date Published:
- Journal Name:
- The Astronomical Journal
- Volume:
- 166
- Issue:
- 1
- ISSN:
- 0004-6256
- Format(s):
- Medium: X Size: Article No. 4
- Size(s):
- Article No. 4
- Sponsoring Org:
- National Science Foundation
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