The newly-discovered white dwarf, designated ZTF J190132.9+145808.7 (ZTF J1901+1458), has a mass of 1.35 solar masses and a radius of 2,140 km (1,330 miles), only slightly larger than the radius of the Moon.
This illustration shows the white dwarf ZTF J190132.9+145808.7 and the Moon. Image credit: Giuseppe Parisi.
ZTF J1901+1458 is located approximately 130 light-years away in the constellation of Aquila.
The white dwarf is about 100 million years old, and has an extreme magnetic field almost 1 billion times stronger than our Sun’s.
It has a rotation period of 6.94 minutes — unusually short for a white dwarf, as white dwarf rotational periods typically are upwards of hours.
“ZTF J1901+1458 is heavy, packing a mass greater than that of our Sun into a body about the size of our Moon,” said Dr. Ilaria Caiazzo, an astronomer in the Division of Physics, Mathematics and Astronomy at Caltech.
“It may seem counterintuitive, but smaller white dwarfs happen to be more massive.”
“This is due to the fact that white dwarfs lack the nuclear burning that keep up normal stars against their own self gravity, and their size is instead regulated by quantum mechanics.”
ZTF J1901+1458 was discovered by the Zwicky Transient Facility (ZTF), which operates at Caltech’s Palomar Observatory.
“No one has systematically been able to explore short-timescale astronomical phenomena on this kind of scale until now. The results of these efforts are stunning,” said Dr. Kevin Burdge, a postdoctoral researcher in the Division of Physics, Mathematics and Astronomy at Caltech.
The astronomers then characterized ZTF J1901+1458 using data from several space- and ground-based telescopes.
The strength of the magnetic field together with the 7-min rotational speed of the object indicated that it was the result of two smaller white dwarfs coalescing into one.
“Many stars orbit around each other in pairs. They grow old together, and if they are both less than 8 solar-masses, they will both evolve into white dwarfs,” the researchers said.
“The new discovery provides an example of what can happen after this phase.”
“The pair of white dwarfs, which spiral around each other, lose energy in the form of gravitational waves and ultimately merge.”
“If the dead stars are massive enough, they explode in what is called a type Ia supernova. But if they are below a certain mass threshold, they combine together into a new white dwarf that is heavier than either progenitor star.”
“This process of merging boosts the magnetic field of that star and speeds up its rotation compared to that of the progenitors.”
“ZTF J1901+1458 took the latter route of evolution; its progenitors merged and produced a white dwarf 1.35 times the mass of our Sun.”
The discovery is reported in a paper in the journal Nature.
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I. Caiazzo et al. 2021. A highly magnetized and rapidly rotating white dwarf as small as the Moon. Nature 595, 39-42; doi: 10.1038/s41586-021-03615-y
