Hertzsprung-Russell diagram showing known (dark blue) and new (light blue) magnetic He-sdOs, as well as stars at 4630Å (purple). Credit: arXiv (2024). DOI: 10.48550/arxiv.2410.02737
An international team of astronomers reports the discovery of three new hot sub-dwarf stars enriched in helium and with powerful magnetic fields. The discovery, made with the Southern African Large Telescope (SALT), is detailed in a research paper published October 3 on the preprint server. arXiv.
Helium-rich subdwarf O stars (He-sdO) are hot compact stars in an evolving pre-white dwarf state. They are the natural result of the merger of double helium white dwarfs (WD). They are dominated by helium and most of them have effective temperatures between 40,000 and 50,000 K.
Observations in recent years have identified a rare class of hot magnetic helium-rich subdwarfs, with magnetic fields ranging from 300 to 500 kG. Their magnetic fields are assumed to be generated during white dwarf merger events. However, it remains unclear why the vast majority of He-sdOs do not exhibit magnetic fields, given that they are also thought to form during such mergers.
Now, a group of astronomers led by Matti Dorsch of the University of Potsdam in Germany reports the detection of three new He-sdOs of the rare magnetic subclass. The discovery was made using SALT’s Robert Stobie Spectrograph (RSS), as part of the ongoing SALT study of hot helium-rich subdwarf stars.
“We identified three new magnetic He-sdO stars among the sample of 592 stars observed with SALT/RSS,” the researchers wrote in the paper.
The new magnetic He-sdOs received the designations J123359.44−674929.11, J125611.42−575333.45 and J144405.79−674400.93. This discovery brings the total number of known hot magnetic subdwarfs to seven stars.
According to the study, the stars detected by Dorsch’s team have magnetic field strengths of around 200 kG. Therefore, their magnetic fields are slightly weaker than those of previously known magnetic He-sdOs.
Observations revealed that J123359.44−674929.11, with a mass of about 0.48 solar masses, is the least massive star of the three reported in the paper. J125611.42−575333.45 and J144405.79−674400.93 have estimated masses of approximately 0.74 and 0.56 solar masses, respectively.
The study also found that the three new He-sdO have comparable radii (between 0.175 and 2.1 solar radii) and effective temperatures (46,000 to 47,680 K).
Analysis of the properties of all known magnetic He-sdOs allowed the authors of the article to draw some conclusions about their origin. They propose that these stars are the result of mergers of helium white dwarfs with hydrogen/helium white dwarfs.
“Our proposal is that magnetic subdwarfs result from the fusion of a He-WD with an H+He-WD. Upon their fusion, the H+He-WD is destroyed and completely mixed. Most of its mass condenses on the surface of the HeWD,” the researchers concluded.
More information:
M. Dorsch et al, Discovery of three magnetic He-sdO with SALT, arXiv (2024). DOI: 10.48550/arxiv.2410.02737
Journal information:
arXiv
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