About 97% of all stars in our Universe are destined to end their lives as white dwarf stars, which represents the final stage in their evolution. Like neutron stars, white dwarfs form after stars have exhausted their nuclear fuel and undergo gravitational collapse, shedding their outer layers to become super-compact stellar remnants. This will be the fate of our Sun billions of years from now, which will swell up to become a red giant before losing its outer layers.
Unlike neutron stars, which result from more massive stars, white dwarfs were once about eight times the mass of our Sun or lighter. For scientists, the density and gravitational force of these objects is an opportunity to study the laws of physics under some of the most extreme conditions imaginable. According to new research led by researchers from Caltech, one such object has been found that is both the smallest and most massive white dwarf ever seen.
The study that describes the research team’s findings appeared in the July 1st issue of the scientific journal Nature. The research was led by Ilaria Caiazzo, the Sherman Fairchild Postdoctoral Scholar Research Associate in Theoretical Astrophysics at Caltech, and included colleagues from Caltech, the University of British Columbia (UBC), UC Santa Cruz, and the Weizmann Institute of Science in Rehovot, Israel.
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This white dwarf, known as ZTF J190132.9+145808.7 (aka. ZTF J1901+1458), is located about 130 light-years from Earth and is estimated to be 1.35 times as massive as our Sun. However, this white dwarf has a stellar radius of about 1810 km (1,125 mi) – slightly larger than the Moon (1,737.4 km; 1,080 mi) – which makes it the smallest and most massive white dwarf ever observed. As Caiazzo explained in a recent press statement from the W.M Keck Observatory:
“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.”
This white dwarf also has an extreme magnetic field, ranging from 600 to 900 MegaGauss (MG) over its entire surface, or roughly 1 billion times stronger than our Sun’s. This magnetic field has one of the fastest rotational periods ever observed in an isolated white dwarf, whipping around the star’s axis once every 6.94 minutes. What’s more, the study of this white dwarf is already offering astronomers insight into how binary systems end their lives.
This curious white dwarf was originally discovered by Kevin Burdge, a postdoctoral scholar at Caltech and a co-author of the recent study. Based on all-sky images taken by the Zwicky Transient Facility (ZTF) at Caltech’s Palomar Observatory, combined with data obtained by the ESA’s Gaia Observatory, it became clear that the white dwarf was also very massive and had a rapid rotation.
Further characterizations were made using the 200-inch Hale Telescope at Palomar, the W. M. Keck Observatory, the Panoramic Survey Telescope and Rapid Response System (PanSTARRS), the ESA’s Gaia Observatory, and NASA’s Neil Gehrels Swift Observatory. Whereas spectra obtained by Keck’s Low-Resolution Imaging Spectrometer (LRIS) revealed signatures of a powerful magnetic field, ultraviolet data from Swift helped constrain the size and mass of the white dwarf.
Between its strong magnetic field and seven-minute rotational speed, Caiazza and her colleagues began to think that ZTF J1901+1458 was the result of two smaller white dwarfs coalescing into one. Roughly 50% of the stars in the observable Universe are binary systems, consisting of two stellar companions that orbit one other. If these stars are less than eight solar masses each, they will evolve into white dwarfs that eventually merge to form a more massive variant.
This process boosts the magnetic field of the resulting white dwarf and speeds up its rotation compared to that of its progenitors. It would also explain how ZTF J1901+1458 manages to concentrate such a considerable mass into a volume slightly more than that of the Moon. In addition, said Caiazzo, they theorize that the remnant could be massive enough to evolve into a neutron star at some point:
“We caught this very interesting object that wasn’t quite massive enough to explode. We are truly probing how massive a white dwarf can be. This is highly speculative, but it’s possible that the white dwarf is massive enough to further collapse into a neutron star. 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.”
If their hypothesis is correct, it may mean that a significant portion of other neutron stars in our galaxy did not start their lives as massive stars, but instead evolved from smaller binary stars. The newfound object’s close proximity to Earth (~130 light-years) and the fact that it is relatively young (100 million years old or so) are indications that similar objects could be common in our galaxy.
In the future, Caiazzo and her colleagues hope to use ZTF to find more white dwarfs like ZTF J1901+1458, as well as more in general. With a census of white dwarfs, scientists will be able to study the population as a whole and determine how many were the result of massive stars experiencing a supernova, and how many were the result of binary companions merging near the end of their lives.
“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?” she said. “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.”