A strange two-faced star baffles astronomers

One of them composed of hydrogen and the other of helium

Astronomers have discovered a white dwarf star that has two faces, one of which is made of hydrogen and the other of helium, as published in the journal ‘Nature’. “The white dwarf’s surface changes completely from one side to the other,” explains Ilaria Caiazzo, a postdoctoral fellow at the California Institute of Technology (Caltech). in the United States, who is leading a new study on these findings. When I show the observations to people, they are blown away.”

White dwarfs are the seething remnants of stars that were once like our Sun. As they age, stars swell and become red giants; eventually, their spongy outer material fluffs off and their cores contract and become fiery, dense white dwarfs. Our Sun will become a white dwarf in about 5 billion years. The new white dwarf, nicknamed Janus after the Roman god of transition, was first discovered by the Zwicky Transient Facility (ZTF), an instrument that scans the sky each night from Caltech’s Palomar Observatory near San Diego. Caiazzo had been searching for highly magnetized white dwarfs, such as the object known as ZTF J1901+1458, which she and her team found earlier using ZTF. One candidate object stood out for its rapid changes in brightness, so Caiazzo decided to investigate further with the CHIMERA instrument at Palomar, as well as HiPERCAM on the Gran Telescopio Canarias in the Spanish Canary Islands. These data confirmed that Jano rotates on its axis every 15 minutes.

Subsequent observations made with the W. M. Keck Observatory atop Maunakea in Hawaii revealed the spectacular double-sided nature of the white dwarf. The team used an instrument called a spectrometer to spread light from the white dwarf into a rainbow of wavelengths that contain chemical signatures. The data revealed the presence of hydrogen when one side of the object was in view (no sign of helium), and only helium when the other side was in view. The team admits to being puzzled by this white dwarf floating alone in space with such drastically different faces, but has come up with some possible theories. One is that we may be witnessing Jano going through a rare phase of white dwarf evolution. “Not all, but some white dwarfs go from having a hydrogen-dominated surface to a helium-dominated one,” Caiazzo explains. We may have caught one of these white dwarfs in the act.”

When white dwarfs form, their heavier elements sink to the core and the lighter ones — hydrogen is the lightest of all — float to the surface. But over time, as white dwarfs cool, the materials mix together. In some cases, hydrogen mixes inside and is diluted in such a way that helium becomes more predominant. Janus may embody this phase of transition, but the question is why the transition is so disjointed, with one side evolving before the other. The answer, according to the scientific team, could lie in magnetic fields.

“Magnetic fields around cosmic bodies tend to be asymmetric, or stronger on one side,” he says. Magnetic fields can prevent mixing of materials. So if the magnetic field is stronger on one side, then that side would have less mixing and therefore more hydrogen.”

Another theory proposed by the team to explain the two faces also depends on magnetic fields. But in this scenario, the fields are thought to change the pressure and density of atmospheric gases. Magnetic fields could reduce the pressure of gases in the atmosphere, allowing the formation of an “ocean” of hydrogen where magnetic fields are strongest,” said James Fuller, co-author of the study and a professor of theoretical astrophysics at Caltech. We don’t know which of these theories is correct, but we can’t think of another way to explain asymmetric sides without magnetic fields.”

More information: Ilaria Caiazzo, A rotating white dwarf shows different compositions on its opposite faces, Nature (2023). DOI: 10.1038/s41586-023-06171-9www.nature.com/articles/s41586-023-06171-9