About 97% of all stars in our universe are destined to end their life as white dwarfs, which is the final stage of their evolution. Like neutron stars, white dwarfs form after stars run out of nuclear fuel and undergo gravitational collapse, repelling 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 into a red giant before losing its outer layers.
In contrast to neutron stars, which emerge from more massive stars, white dwarfs once had about eight times the mass of our sun or less. For scientists, the density and gravitational force of these objects provide an opportunity to study the laws of physics in some of the most extreme conditions imaginable. One such object has been found to be both the smallest and the most massive white dwarf ever seen, according to new research led by researchers from Caltech.
The study, describing the research team’s findings, appeared in the July 1 issue of Nature. The research was led by Ilaria Caiazzo, 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.
Artist’s impression of the white dwarf ZTF J1901 + 1458 over the moon in this artist’s impression; in reality, the white dwarf is 130 light-years away in the constellation of eagles. Photo credit: Giuseppe Parisi
This white dwarf, known as ZTF J190132.9 + 145808.7 (also known as 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 1,810 km (1,125 miles) – slightly larger than the Moon (1,737.4 km; 1,080 miles) – making it the smallest and most massive white dwarf ever observed. As Caiazzo explained in a recent press release from the WM Keck Observatory:
“It may seem counter-intuitive, but smaller white dwarfs happen to be more massive. This is because white dwarfs lack the nuclear combustion that normal stars sustain against their own gravity, and their size is instead regulated by quantum mechanics. “
This white dwarf also has an extreme magnetic field of 600 to 900 MegaGauss (MG) over its entire surface, or about 1 billion times stronger than that of our sun. This magnetic field has one of the fastest periods of rotation ever observed in an isolated white dwarf, whipping around the star’s axis once every 6.94 minutes. In addition, the study of this white dwarf is already providing astronomers with insights into how binary systems end their lives.
This strange white dwarf was originally discovered by Kevin Burdge, a postdoctoral fellow at Caltech and co-author of the current study. Based on all-sky images from the Zwicky Transient Facility (ZTF) at Caltech’s Palomar Observatory, combined with data from ESA’s Gaia Observatory, it became clear that the white dwarf was also very massive and had rapid rotation.
Artist’s impression of a white dwarf from the surface of an orbiting exoplanet. Image source: Madden / Cornell University
Further characterizations were carried out using the 200-inch Hale Telescope in Palomar, the WM Keck Observatory, the Panoramic Survey Telescope and Rapid Response System (PanSTARRS), the ESA’s Gaia Observatory and NASA’s Neil Gehrels Swift Observatory . While spectra from Keck’s Low-Resolution Imaging Spectrometer (LRIS) showed signatures of a strong magnetic field, ultraviolet data from Swift helped limit the size and mass of the white dwarf.
Between its strong magnetic field and a seven-minute rotation speed, Caiazza and her colleagues began to think that ZTF J1901 + 1458 was the result of two smaller white dwarfs merging into one. About 50% of the stars in the observable universe are double systems consisting of two stellar companions orbiting each other. When these stars each have less than eight solar masses, they evolve into white dwarfs, which eventually merge into a more massive variant.
This process amplifies the magnetic field of the resulting white dwarf and speeds up its rotation compared to its ancestors. It would also explain how ZTF J1901 + 1458 manages to concentrate such a considerable mass in a volume slightly above that of the moon. In addition, according to Caiazzo, they theorize that the remnant could be massive enough to eventually evolve into a neutron star:
“We caught this very interesting object that wasn’t massive enough to explode. We’re really investigating how massive a white dwarf can be. This is highly speculative, but it is possible that the white dwarf is massive enough to continue to collapse into a neutron star. It is so massive and dense that in its core, electrons are trapped by protons in nuclei to form neutrons. Since the pressure of the electrons works against gravity and keeps the star intact, the nucleus collapses when a sufficiently large number of electrons are removed. “
If your hypothesis is correct, it may mean that a significant fraction of other neutron stars in our galaxy did not begin life as massive stars, but instead evolved from smaller binary stars. The proximity of the newly discovered object to Earth (~ 130 light years) and the fact that it is relatively young (around 100 million years old) are indications that similar objects could be abundant in our galaxy.
In the future, Caiazzo and her colleagues hope to find more white dwarfs like ZTF J1901 + 1458 and more in general with ZTF. With a white dwarf count, scientists will be able to study the population as a whole and determine how many of massive stars saw a supernova and how many of binary companions merging towards the end of their lives.
“There are so many questions to answer, like what is the merging rate of white dwarfs in the galaxy and is this enough to explain the number of Type Ia supernovae?” She said. “How is a magnetic field generated during these powerful events and why is there such a variety of magnetic field strengths in white dwarfs? Finding a large population of merger white dwarfs will help us answer all of these questions and more. “
Further reading: WM Keck observatory, nature