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Newfound White Dwarf Has Different Compositions on Its Opposite Faces

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ZTF J203349.8+322901.1, nicknamed Janus after the two-faced Roman god of transition, is a transitioning white dwarf with two faces: one side of its atmosphere is dominated by hydrogen and the other one by helium.

An artist’s impression of the two-faced white dwarf ZTF J203349.8+322901.1. Image credit: K. Miller, Caltech / IPAC.

ZTF J203349.8+322901.1 (ZTF J1901+1458 for short) was discovered in 2019 during a search for periodically variable white dwarfs with the Zwicky Transient Facility (ZTF), an instrument that scans the skies every night from Caltech’s Palomar Observatory.

This white dwarf is located approximately 130 light-years away in the constellation of Aquila.

It is about 100 million years old, has a mass of 1.35 solar masses and a radius of 2,140 km (1,330 miles).

ZTF J1901+1458 stood out for its rapid changes in brightness, so Caltech astronomer Ilaria Caiazzo and colleagues decided to investigate further with the CHIMERA instrument at Palomar, as well as HiPERCAM on the Gran Telescopio Canarias.

Those data confirmed that ZTF J1901+1458 is rotating on its axis every 15 minutes.

Subsequent observations made with the W.M. Keck Observatory revealed the dramatic double-faced nature of the white dwarf.

The astronomers used an instrument called a spectrometer to spread the light of the white dwarf into a rainbow of wavelengths that contain chemical fingerprints.

The data revealed the presence hydrogen when one side of the object was in view (with no signs of helium), and only helium when the other side swung into view.

What would cause a white dwarf floating alone in space to have such drastically different faces?

The authors acknowledge they are baffled but have come up with some possible theories.

One idea is that we may be witnessing ZTF J1901+1458 undergoing a rare phase of white dwarf evolution.

“Not all, but some white dwarfs transition from being hydrogen- to helium-dominated on their surface. We might have possibly caught one such white dwarf in the act,” Dr. Caiazzo said.

After white dwarfs are formed, their heavier elements sink to their cores and their lighter elements — hydrogen being the lightest of all — float to the top. But over time, as the white dwarfs cool, the materials are thought to mix together.

In some cases, the hydrogen is mixed into the interior and diluted such that helium becomes more prevalent.

ZTF J1901+1458 may embody this transition phase, but one pressing question is: why is the transition happening in such a disjointed way, with one side evolving before the other?

The answer, according to the team, may lie in magnetic fields.

“Magnetic fields around cosmic bodies tend to be asymmetric, or stronger on one side,” Dr. 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 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 the atmospheric gases.

“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,” said Caltech Professor James Fuller.

“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.”

The study was published in the journal Nature.


I. Caiazzo et al. A rotating white dwarf shows different compositions on its opposite faces. Nature, published online July 19, 2023; doi: 10.1038/s41586-023-06171-9

Source : Breaking Science News

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