‘Dead stars’ may have surprisingly healthy appetites

“Dead stars” known as white dwarfs have already shown the potential to help us better understand how planetary systems, including our own solar system, take shape and evolve. Now, research from the University of Michigan and the University of Colorado Boulder suggests that potential could be even greater than anticipated.

Although the galaxy’s white dwarfs are too dim or distant for us to see with the naked eye, astronomers have known about them for more than a century. Researchers have also found that, despite their “dead” designation, white dwarfs can still suck in material from comets, asteroids and even planets that come too close. The white dwarf atomizes these materials into their elemental building blocks, which researchers can observe to extract important glimpses into a planetary system’s composition and history that would otherwise be inaccessible.

A new study has shown that white dwarfs could be gobbling up such material—referred to as white dwarf pollution—at a much higher rate than expected. The study, led by Aster Taylor of U-M and Dang Pham of CU Boulder, has been accepted by The Astrophysical Journal and is currently available on the arXiv preprint server.

“This means that there may be significantly more pollution on white dwarfs than we have measured so far, and that the remnants of planetary systems may be even more common around these dead stars,” said Taylor, a Fannie and John Hertz Fellow in the U-M Department of Astronomy. “And this is telling us that planetary systems are still pretty active even after the death of their host star.”

White dwarf pollution

Stars die when they burn through their nuclear fuel, but what those final stages look like varies dramatically depending on a star’s mass. The most massive stars erupt as supernovae, but smaller ones, like our sun and about 97% of the stars in the Milky Way, shed their outer layers without such fireworks.

The remaining cores are white dwarfs, which still have masses comparable to the sun. But when nuclear reactions cease, so do the forces opposing the gravity of all that mass and, as a result, the dead star collapses. White dwarfs cram their mass into a volume about the size of Earth.

Without nuclear fusion, white dwarfs no longer burn, but they are still hot enough to glow. Astronomers can spot and characterize them with tools like telescopes and spectrometers. These tools have also enabled astronomers to detect white dwarf pollution, which is surprisingly common, said Pham, a postdoctoral researcher at CU Boulder.

Because white dwarfs are so dense and the polluting elements are so heavy, one could reasonably expect the pollution to sink into dead stars and disappear before astronomers could measure it. But astronomers have observed pollution in about 50% of known white dwarfs.

“It’s like going around to look at every bathtub in the U.S. and finding a metal bar floating in half of them,” Pham said.

Magnetic fields may hide more

That means white dwarfs must be taking in an appreciable amount of material at a significant rate: The inflowing material has to at least replace what’s sinking. But Pham and Taylor’s new model suggests that the rate could be even higher than anticipated.

Working with Tim Cunningham, a NASA Hubble Fellow at the Harvard & Smithsonian Center for Astrophysics, the researchers examined the effect of a white dwarf’s magnetic fields on the pollution. They found that the field concentrates the pollution into small spots around the white dwarf’s magnetic poles.

“It’s actually a very similar process to auroras on Earth. The sun sends charged particles to Earth, they follow Earth’s magnetic field lines and then they make a spot on Earth’s atmosphere, which is the aurora,” Pham said. “With the white dwarf, instead of materials coming from the sun, you get materials from the planetary system. But we’re basically calculating the size of auroras on the white dwarfs, which I think is a very cool thing.”

If the material is concentrated in a couple of small spots, that would mean much more of it has to be flowing in than if it were spread out over the entire surface of the white dwarf. Spotting such spots with current observational techniques, however, is incredibly difficult, Taylor said, so the theory is difficult to support with current techniques. That said, there are two white dwarfs on record that appear to support it.

A bigger debris puzzle

The study also raises another issue tied to how much planetary material needs to be surrounding white dwarfs to support the team’s calculated rate. It would require about 100 times more material than previously expected. While there is definitely wiggle room in that expectation, a factor of 100 is a stretch, Taylor said.

“At the very least, I think this says we have to update our calibrations about how many objects we expect to be out there and how we expect the dynamics to play out,” Taylor said. “What we’ve done sort of lets you squint at possible solutions, but, as of yet, there’s nothing definitive. I think it’s very much an open problem that we’ve introduced.”

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Lisa Lock

BA art history, MA material culture. Former museum editor, paramedic, and transplant coordinator. Editing for Science X since 2021.

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Robert Egan

Bachelor’s in mathematical biology, Master’s in creative writing. Well-traveled with unique perspectives on science and language.

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