In a world first, astronomers discover 'mind-blowing' two-faced star

"The surface of the white dwarf completely changes from one side to the other," explained Ilaria Caiazzo, a postdoctoral scholar at Caltech who led the new study. "When I show the observations to people, they are blown away."

The newly-discovered white dwarf, dubbed Janus after the two-faced Roman god of transition, was discovered using the Zwicky Transient Facility (ZTF) at Caltech's Palomar Observatory near San Diego.

Caiazzo found Janus while searching for highly magnetized white dwarfs. During the search, one candidate stood out due to the fact its brightness was rapidly changing, leading the researcher to perform follow-up observations using the CHIMERA instrument at Palomar and the HiPERCAM on the Gran Telescopio Canarias in Spain's Canary Islands.

The follow-up observations showed that Janus is rotating on its axis every 15 minutes. Further observations using the W. M. Keck Observatory in Hawai'i then revealed the unusual twin faces of the white dwarf. Using a spectrometer, the team revealed the presence of hydrogen (and no helium) on one side and only helium on the other side.

Why does Janus have two faces?

The team behind the discovery acknowledged that they don't know the cause behind this never-before-seen phenomenon, though they have provided a few hypotheses.

One theory is that Janus may be undergoing a rare phase of white dwarf evolution. "Not all, but some white dwarfs transition from being hydrogen- to helium-dominated on their surface," Caiazzo said. "We might have possibly caught one such white dwarf in the act."

As a white dwarf cools over time, the hydrogen on its surface sometimes mixes into the star's interior and helium becomes more prevalent. Magnetic fields may be causing this evolution to take place on one side more than the other.

"Magnetic fields around cosmic bodies tend to be asymmetric, or stronger on one side," Caiazzo explained. "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."

Next, the team hopes to find more stars like Janus to help it better understand the mystery of the two-faced white dwarf star Janus. "ZTF is very good at finding strange objects," Caiazzo explained, adding that other observatories will also help in the search.

Study Abstract:

White dwarfs, the extremely dense remnants left behind by most stars after their death, are characterized by a mass comparable to that of the Sun compressed into the size of an Earth-like planet. In the resulting strong gravity, heavy elements sink towards the centre and the upper layer of the atmosphere contains only the lightest element present, usually hydrogen or helium Several mechanisms compete with gravitational settling to change a white dwarf’s surface composition as it cools, and the fraction of white dwarfs with helium atmospheres is known to increase by a factor of about 2.5 below a temperature of about 30,000 kelvin therefore, some white dwarfs that appear to have hydrogen-dominated atmospheres above 30,000 kelvin are bound to transition to be helium-dominated as they cool below it. Here we report observations of ZTF J203349.8+322901.1, a transitioning white dwarf with two faces: one side of its atmosphere is dominated by hydrogen and the other one by helium. This peculiar nature is probably caused by the presence of a small magnetic field, which creates an inhomogeneity in temperature, pressure or mixing strength over the surface. ZTF J203349.8+322901.1 might be the most extreme member of a class of magnetic, transitioning white dwarfs—together with GD 323, a white dwarf that shows similar but much more subtle variations. This class of white dwarfs could help shed light on the physical mechanisms behind the spectral evolution of white dwarfs.