Astronomers Discover Smallest, Most Massive White Dwarf Ever Seen

Astronomers have discovered the smallest and most massive white dwarf ever seen.

The discovery was made by the Zwicky Transient Facility (ZTF), which operates at Caltech’s Palomar Observatory; two Hawaiʻi telescopes – WM Keck Observatory on Maunakea and University of Hawaiʻi 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.

The smoldering cinder, which 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,” said Ilaria Caiazzo, the Sherman Fairchild Postdoctoral Scholar Research Associate in Theoretical Astrophysics at Caltech and lead author of the new study appearing in the July 1 issue of the journal Nature. “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.”

While the earth’s sun is alone in space without a stellar partner, many stars orbit around each other in pairs. The stars grow old together, and if they are both less than eight solar-masses, they will both evolve into white dwarfs.

The new discovery provides an example of what can happen after this phase. The pair of white dwarfs, which spiral around each other, lose energy in the form of gravitational waves and ultimately merge. If the dead stars are massive enough, they explode in what is called a type Ia supernova. But if they are below a certain mass threshold, they combine together into a new white dwarf that is heavier than either progenitor star. This process of merging boosts the magnetic field of that star and speeds up its rotation compared to that of the progenitors.

Astronomers say that the newfound tiny white dwarf, named ZTF J1901+1458, took the latter route of evolution; its progenitors merged and produced a white dwarf 1.35 times the mass of the earth’s sun. The white dwarf has an extreme magnetic field almost one billion times stronger than the sun’s and whips around on its axis at a frenzied pace of one revolution every seven minutes (the zippiest white dwarf known, called EPIC 228939929, rotates every 5.3 minutes).

“We caught this very interesting object that wasn’t quite massive enough to explode,” says Caiazzo. “We are truly probing how massive a white dwarf can be.”

What’s more, Caiazzo and her collaborators think that the merged white dwarf may be massive enough to evolve into a neutron-rich dead star, or neutron star, which typically forms when a star much more massive than the earth’s sun explodes in a supernova.

“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.”

If this neutron star formation hypothesis is correct, it may mean that a significant portion of other neutron stars take shape in this way. The newfound object’s close proximity (about 130 light-years away) and its young age (about 100 million years old or less) indicate that similar objects may occur more commonly in our galaxy.

White dwarfs are the collapsed remnants of stars that were once about eight times the mass of the earth’s sun or lighter. The sun, for example, after it first puffs up into a red giant in about five billion years, will ultimately slough off its outer layers and shrink down into a compact white dwarf. About 97% of all stars become white dwarfs.