Hawaii, Maunakea & Haleakala -- Astronomers have found the largest and most massive white dwarf yet. The smoldering, fusion of two smaller white dwarfs created a smoldering, cinder that is about half the size of the Moon. Ilaria Caiazzo (Sherman Fairchild Postdoctoral Scholar Research Assistant in Theoretical Astrophysics at Caltech) was the lead author of the study, published in Nature's July 1 issue. It may seem paradoxical, but smaller white dwarfs are actually more massive. White dwarfs are not capable of producing the nuclear burning necessary to keep normal stars from collapsing against themselves, and instead their size is controlled by quantum mechanics.The Zwicky Transient Facility (or ZTF) made the discovery at Caltech's Palomar Observatory. Two Hawai'i telescopes, W. M. Keck Observatory, Maunakea, Hawai'i Islands and University of Hawai'i Institute for Astronomy's Pan-STARRS on Haleakala on Maui, helped to characterize the star. They also used the 200-inch Hale Telescope at Palomar and the European Gaia satellite observatory.White dwarfs are stars that have been weakened and become smaller than our Sun. After it has inflated into a red giant for about 5 billion years, our Sun will eventually slough away its outer layers to become a compact white dwarf. White dwarfs are about 97 percent of all stars.Our Sun orbits alone in space, but many stars orbit each other in pairs. If they have less than eight solar masses, the stars will grow old together and they will become white dwarfs.This new discovery is an example of what could happen in the next phase. The white dwarfs spiral around one another, losing energy through gravitational waves, and eventually merging. If the dead stars are large enough, they can explode in a so-called type Ia supernova. If they are less than a certain mass threshold they will combine to form a new white dwarf, which is heavier than the progenitor stars. The merging process increases the star's magnetic field and speeds up its orbit compared to the progenitors.Astronomers believe that the tiny white dwarf discovered by astronomers, ZTF J1901+1458, evolved via the second route. Its progenitors combined to create a white dwarf with a mass 1.35 times greater than our Sun. The white dwarf's magnetic field is almost one billion times stronger than the Sun's. It spins on its axis at an alarming rate of one revolution every seven min. EPIC 22893992929, which is the fastest known white dwarf, rotates every 5.3 seconds.AdvertisementCaiazzo says, "We found this very interesting object that wasn’t quite large enough to explode." "We are really probing the size of a white dwarf."Caiazzo and her colleagues believe that the merged white dwarf could be large enough to become a neutron rich dead star or neutron star. These are formed when a star more massive than our Sun explodes during a supernova.Caiazzo says that although this is highly speculational, it is possible that the white dwarf could be massive enough to collapse further into a neutron-star. It is so dense and massive that electrons in its core are being captured by protons within nuclei to make neutrons. The force of gravity is pushed against by electron pressure, which keeps the star intact. When enough electrons are removed, the core will collapse.This neutron star formation hypothesis may be correct. It could mean that some other neutron stars form in this manner. The object is close to Earth (130 light-years away), and it's young age (100 million years or less) suggest that similar objects could be more common in our galaxy.FAST AND MAGNETICAfter looking through ZTF all-sky images, Caiazzo's colleague Kevin Burdge discovered the white dwarf. When analyzed with data from Gaia this particular white dwarf stood out because it was very large and had a rapid spin.Advertisement"No one has been able to systematically explore short-timescale phenomena in astronomy on such a scale before now." These results are amazing," Burdge says. Burdge was the leader of the 2019 team that discovered white dwarfs zipping around one another every seven minutes.After analyzing the spectrum of the star with Keck Observatory's Low Resolution Imaging Spectrometer, (LRIS), Caiazzo realized she had discovered something "very special" and was convinced that it was the result of a powerful magnetic field. It was due to two smaller white dwarfs merging into one. The object's seven-minute rotational speed and strength indicated this.Swift data, which monitors ultraviolet light, was instrumental in determining the white dwarf's size and mass. ZTF J1901+1458 has a diameter measuring 2,670 miles. This puts it in the lead for the smallest white dwarf known.Caiazzo plans to use ZTF in the future to search for more white dwarfs similar to this one and to study the entire population. There are many questions to be answered, including what is the rate at which white dwarf mergers occur in the galaxy and is this enough to explain the high number of supernovae of type Ia. What is the magnetic field created by these powerful events? And why are there so many variations in magnetic field strengths between white dwarfs and other white dwarfs?" We can answer these questions by finding a large number of white dwarfs that were born through mergers.
