Asymmetric magnetic fields may explain two-faced white dwarf

An artist’s impression of an unusual white dwarf featuring one side dominated by hydrogen and the other by helium, giving the star a two-faced appearance. Image: K. Miller, Caltech/IPAC

Astronomers have found a white dwarf with two faces, one side dominated by hydrogen and the other by helium. Appropriately enough, they’ve dubbed it Janus after the two-faced Roman god of transition and duality.

“The surface of the white dwarf completely changes from one side to the other,” said Ilaria Caiazzo, a postdoctoral scholar at Caltech who led a study describing the findings in the journal Nature. “When I show the observations to people, they are blown away.”

White dwarfs are the remnants of stars like our Sun that have used up their nuclear fuel, swelling to become red giants before blowing off their outer layers. The cores then collapse, forming slowly cooling planet-size dwarf stars.

Heavier elements sink into the interior while lighter elements like hydrogen float to the top. Over time, as the dwarf cools, the materials mix together and in some cases, helium can become more prevalent.

The Janus dwarf was discovered by the Zwicky Transient Facility at Caltech’s Palomar Observatory near San Diego, California. Caiazzo was searching for highly magnetised white dwarfs when her team took a closer look at a candidate that showed rapid brightness changes.

Astronomers say the action of asymmetric magnetic fields may play a role in Janus’ two-faced appearance, affecting how gases mix in the white dwarf’s outer layers. Image: K. Miller, Caltech/IPAC

Using the CHIMERA instrument at Palomar and the HiPERCAM on the Gran Telescopio Canarias in the Canary Islands, they found it was rotating on its axis once every 15 minutes. Subsequent spectroscopic observations using the W.M. Keck Observatory in Hawaii showed one side was dominated by hydrogen and the other side by helium.

Baffled, the team came up with two possible explanations, both involving the dwarf’s magnetic field.

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

Another possibility: the magnetic fields could change the pressure and density of the gases. Said co-author James Fuller, professor of theoretical astrophysics at Caltech: “The magnetic fields may lead to lower gas pressures in the atmosphere, and this may allow a hydrogen ‘ocean’ to form where the magnetic fields are strongest.”

“We don’t know which of these theories are correct, but we can’t think of any other way to explain the asymmetric sides without magnetic fields.”