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.
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…
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