The two-faced star opens
(Nanowerk News) For the first time for a white dwarf, the burnt-out core of a dead star, astronomers have found that at least one member of this cosmic family is two-faced. One side of a white dwarf is made up of hydrogen, while the other side is made up of helium.
“The surface of the white dwarf actually turns from side to side,” said Ilaria Caiazzo, a postdoctoral scholar at Caltech who led a new study of the findings in the journal. Natural (“Rotating white dwarf shows a different composition on opposite sides”). “When I show people observations, they are blown away.”
White dwarfs are the hot remnants of stars that were once like our sun. As stars age, they expand to become red giants; eventually, its silky outer material is blown away and its core shrinks into a dense, smoldering white dwarf. Our sun will evolve into a white dwarf in about 5 billion years.
The newly discovered white dwarf, nicknamed Janus after the two-faced Roman god of transition, was originally discovered by the Zwicky Transient Facility (ZTF), an instrument that scans the sky every night from Caltech’s Palomar Observatory near San Diego. Caiazzo has been searching for a highly magnetic white dwarf, such as the object known as ZTF J1901+1458, which he and his team discovered previously using ZTF. One candidate object stood out for its rapid change in brightness, so Caiazzo decided to investigate further with the CHIMERA instrument in Palomar, as well as HiPERCAM on Gran Telescopio Canarias in Spain’s Canary Islands. The data confirms that Janus rotates on its axis every 15 minutes.
Subsequent observations made with the WM Keck Observatory atop Maunakea in Hawaii revealed the dramatic nature of two-faced white dwarfs. The team used an instrument called a spectrometer to scatter the white dwarf’s light into a rainbow of wavelengths that contain chemical fingerprints. The data revealed the presence of hydrogen when one side of the object was visible (with no sign of helium), and only helium when the other side was visible.
What causes a white dwarf floating alone in space to have such a different face? The team admit they are confused but have come up with a few possible theories. One idea is that we might be witnessing Janus go through a rare white dwarf phase of evolution.
“Not all, but some white dwarfs are transitioning from hydrogen to helium dominance on their surface,” explains Caiazzo. “We just might catch one of the white dwarfs in action.”
After a white dwarf forms, its heavier elements sink to its core and lighter elements—the lightest hydrogen—float to the top. But over time, as the white dwarf cooled, the materials were thought to mix together. In some cases, hydrogen is mixed into the interior and diluted in such a way that helium becomes more common. Janus may have embodied this transitional phase, but one pressing question is: why did the transitions occur in such disjointed ways, with one side developing before the other?
The answer, according to the science team, may lie in magnetic fields.
“The magnetic fields around cosmic bodies tend to be asymmetrical, or stronger on one side,” explains Caiazzo. “The magnetic field can prevent the mixing of materials. So if the magnetic field is stronger on one side, then that side will have less mixing and thus more hydrogen.”
Another theory put forward by the team to explain the two faces also relies on magnetic fields. But in this scenario, the field is assumed to change the pressure and density of the atmospheric gases.
“Magnetic fields can lead to lower gas pressure in the atmosphere, and this may allow hydrogen ‘oceans’ to form where the magnetic field is strongest,” said co-author James Fuller, professor of theoretical astrophysics at Caltech. “We don’t know which theory is correct, but we can think of no other way to explain the asymmetrical side without a magnetic field.”
To help solve the mystery, the team hopes to find more Janus-like white dwarfs with the ZTF sky survey. “ZTF is very good at finding strange objects,” says Caiazzo. Future surveys, such as those to be conducted by the Vera C. Rubin Observatory in Chile, he said, would make the search for variable white dwarfs even easier.
