An international team of astronomers has reported the discovery of three new hot helium-rich subdwarf stars with strong magnetic fields. The discovery, made using the South African Large Telescope (SALT), is detailed in a research paper:
Published on the preprint server on October 3 arXiv.

Helium-rich subdwarf O stars (He-sdOs) are hot, compact stars in a pre-white dwarf evolutionary state. These are the natural result of the binary merger of helium white dwarfs (WD). Helium predominates, and most have effective temperatures between 40,000 and 50,000 K.

Observations in recent years have revealed a rare class of magnetic helium-rich hot subdwarfs with magnetic fields in the range of 300 to 500 kHz. It is assumed that their magnetic fields are produced in white dwarf merger events. However, it is still unclear why the vast majority of He-sdOs do not exhibit magnetic fields, as it is believed that they are also formed during such mergers.

Now a team of astronomers led by Matti Dorsch of the University of Potsdam in Germany reports the discovery of three new He-sdO rare magnetic subclasses. The discovery was made as part of SALT’s ongoing search for helium-rich, hot subdwarf stars with SALT’s Robert Stobie Spectrograph (RSS).

“We identified three new magnetic He-sdO stars among a sample of 592 stars observed with SALT/RSS,” the researchers wrote in the paper.

The newly discovered magnetic He-sdO was named J123359.44−674929.11, J125611.42−575333.45, and J144405.79−674400.93. This discovery brings the total number of known magnetic hot subdwarfs to seven stars.

According to the research, the magnetic field strengths of the stars discovered by the Dorsch team are around 200 kHz. Therefore, their magnetic fields are slightly weaker than those of previously known magnetic He-sdOs. Observations have shown that J123359.44−674929.11 is the least massive star of the three stars discussed in the article, with a mass of approximately 0.48 solar masses. The masses of J125611.42−575333.45 and J144405.79−674400.93 are roughly equal to approximately 0.74 and 0.56 solar masses, respectively.

The study also found that all three new He-sdOs have comparable radii (between 0.175 and 2.1 solar radii) and effective temperatures (46,000-47,680 K).

Analysis of the properties of all known magnetic He-sdO allowed the authors of the article to draw some conclusions about their origin. They suggest that these stars are the result of the merger of helium white dwarfs and hydrogen-helium white dwarfs.

“Our proposal is that magnetic subdwarfs arise from the merger of He-WD with H+He-WD. When they merge, H+He-WD collapses and mixes completely. The researchers noted that most of its mass is concentrated on the surface of HeWD.”