Smallest, most massive white dwarf on verge of collapse to neutron star

An artist’s impression of a record-setting white dwarf with 1.35 times the mass of the Sun, shown in comparison with Earth’s Moon. Image: Giuseppe Parisi

Astronomers have found the smallest, most massive white dwarf ever seen, a hugely compressed body 130 light years from Earth that packs more mass than the Sun into a slowly cooling cinder just slightly larger than Earth’s Moon.

Known as ZTF J1901+1458, the white dwarf is spinning at seven revolutions per minute with a magnetic field a billion times stronger than the Sun’s. It is just 4,300 kilometres (2,670 miles) across, forming when two less massive white dwarfs spiralled together and merged less than 100 million years ago.

“We caught this very interesting object that wasn’t quite massive enough to explode,” said Ilaria Caiazzo, a postdoc in theoretical astrophysics at Caltech and author of a study in Nature. “We are truly probing how massive a white dwarf can be.”

With a mass of 1.35 times that of the Sun, the dwarf is just below the threshold needed to trigger a type 1a supernova. Even so, electrons and protons may be merging in the enormous pressures at the star’s core, possibly setting the stage for a dramatic future transformation.

“This is highly speculative, but it’s possible that the white dwarf is massive enough to further collapse into a neutron star,” says Caiazzo. “It is so massive and dense that, in its core, electrons are being captured by protons in nuclei to form neutrons. Because the pressure from electrons pushes against the force of gravity, keeping the star intact, the core collapses when a large enough number of electrons are removed.”

Stars like the Sun balance the inward pull of gravity with the outward radiation pressure produce by nuclear fusion in the core. When the nuclear fuel is consumed, the core collapses and the star’s outer layers are blown away.

For stars with up to about eight times the mass of the Sun, the core collapse will stop at the white dwarf stage thanks to quantum limits on how tightly electrons can be packed together. This is the fate of about 97 percent of all stars.

But if a sun is massive enough, gravity will overcome the “electron degeneracy” pressure, and the core can implode to the point where protons and electrons are forced together to form an ultra-dense neutron star. For stars with even more mass, gravity overwhelms all quantum resistance and a black hole is formed.

ZTF J1901+1458 was found by the Zwicky Transient Facility at Caltech’s Palomar Observatory. Caiazzo plans to use ZTF to search for more white dwarfs that might have formed through similar mergers.

“There are so many questions to address, such as what is the rate of white dwarf mergers in the galaxy, and is it enough to explain the number of type Ia supernovae?” Caiazzo wonders. “How is a magnetic field generated in these powerful events, and why is there such diversity in magnetic field strengths among white dwarfs?

“Finding a large population of white dwarfs born from mergers will help us answer all these questions and more.”