Astronomers have discovered the smallest and most massive white dwarf ever discovered. They said it’s a star that’s both small and massive, and it’s on the verge of collapsing. A white dwarf is a star that has evolved. One day, our sun will merge with another. ZTF J1901+1458 is the name given to this white dwarf. It is approximately 130 light-years away, in the direction of the constellation Aquila the Eagle. This very massive white dwarf is thought to have formed from the merger of two less massive white dwarfs.
Hawai’i’s Maunakea and Haleakala volcanoes Astronomers have discovered the world’s smallest and most massive white dwarf. The smoldering cinder, formed when two less massive white dwarfs merged, is heavy, “packing a mass greater than that of our Sun into a body about the size of our Moon,” according to Ilaria Caiazzo, the Sherman Fairchild Postdoctoral Scholar Research Associate in Theoretical Astrophysics at Caltech and lead author of the new study published in the journal Nature on July 1.
“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 keeps up normal stars against their own self-gravity, and their size is instead regulated by quantum mechanics.”
The discovery was made by the Zwicky Transient Facility, or ZTF, which operates at Caltech’s Palomar Observatory; two Hawai’i telescopes — W. M. Keck Observatory on Maunakea, Hawai’i Island, and University of Hawai’i Institute for Astronomy’s Pan-STARRS (Panoramic Survey Telescope and Rapid Response System) on Haleakala, Maui — helped characterize the dead star, along with the 200-inch Hale Telescope at Palomar, the European Gaia space observatory, and NASA’s Neil Gehrels Swift Observatory.
Astronomers have identified an extremely magnetized and rapidly rotating ultra-massive white dwarf. Several telescopes characterized the dead star.
White dwarfs are the shattered remains of stars that were once eight times the mass of our Sun or less. Our Sun, for example, will eventually slough off its outer layers and shrink down to a compact white dwarf after first puffing up into a red giant in about 5 billion years. Approximately 97 percent of all stars die and become white dwarfs.
While our Sun is the only star in space without a stellar companion, many stars orbit in pairs. The stars age together, and if their masses are less than eight solar masses, they will both become white dwarfs.
The new discovery exemplifies what can happen after this stage. The pair of white dwarfs lose energy in the form of gravitational waves as they spiral around each other and eventually merge. If the dead stars are massive enough, they will explode in a type Ia supernova. If their combined mass is less than a certain threshold, they merge to form a new white dwarf that is heavier than either progenitor star. This merging process increases the magnetic field of that star and speeds up its rotation in comparison to the progenitors.
According to astronomers, the newly discovered tiny white dwarf, ZTF J1901+1458, evolved via the latter route, with its progenitors merging to produce a white dwarf 1.35 times the mass of our Sun. The white dwarf has a magnetic field nearly one billion times stronger than our Sun and spins on its axis at a dizzying rate of one revolution every seven minutes (the zippiest white dwarf known, called EPIC 228939929, rotates every 5.3 minutes).
“We captured this very interesting object that wasn’t quite massive enough to explode,” Caiazzo explains. “We’re really testing how big a white dwarf can get.” Furthermore, Caiazzo and her colleagues believe that the merged white dwarf may be massive enough to evolve into a neutron-rich dead star, or neutron star, which is formed when a star much larger than our Sun explodes in a supernova.
“It’s highly speculative, but it’s possible that the white dwarf is massive enough to collapse further into a neutron star,” Caiazzo says. “It is so massive and dense that electrons in its core are captured by protons in nuclei to form neutrons. The core collapses when a large enough number of electrons are removed because the pressure from electrons pushes against the force of gravity, keeping the star intact.”
If this neutron star formation hypothesis is correct, it could imply that a large proportion of other neutron stars form in this manner. The newfound object’s close proximity (approximately 130 light-years away) and young age (approximately 100 million years old or less) suggest that similar objects may occur more frequently in our galaxy.
MAGNETIC AND FAST
After searching through all-sky images captured by ZTF, Caiazzo’s colleague Kevin Burdge, a postdoctoral scholar at Caltech, discovered the white dwarf. When combined with Gaia data, this particular white dwarf stood out for being very massive and rotating at a rapid rate.
“Until now, no one has been able to investigate short-timescale astronomical phenomena on this scale in a systematic manner. These efforts have yielded stunning results “Burdge led the team that discovered a pair of white dwarfs zipping around each other every seven minutes in 2019.
The team then analyzed the star’s spectrum with Keck Observatory’s Low-Resolution Imaging Spectrometer (LRIS), and Caiazzo was struck by the signatures of a very powerful magnetic field, realizing she and her team had discovered something “very special,” as she puts it. The strength of the magnetic field, combined with the object’s seven-minute rotational speed, indicated that it was the result of two smaller white dwarfs merging into one.
Swift data, which measures ultraviolet light, assisted in determining the size and mass of the white dwarf. ZTF J1901+1458 is the smallest known white dwarf, with a diameter of 2,670 miles, edging out previous record holders RE J0317-853 and WD 1832+089, both of which have diameters of around 3,100 miles.
Caiazzo hopes to use ZTF in the future to find more white dwarfs like this one, as well as to study the population as a whole. “There are numerous questions to be answered, such as what the rate of white dwarf mergers in the galaxy is and whether it is sufficient to explain the number of type Ia supernovae. Why is there such variation in magnetic field strengths among white dwarfs, and how is a magnetic field generated in these powerful events? Finding a large population of white dwarfs resulting from mergers will help us answer these and other questions.”
