Study: The Effects of Magnetic Accretion on the Spatial Extent of White Dwarf Pollution (arXiv: 2607.20747)
"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 is showing that potential could be even greater than anticipated.
Although the galaxy's white dwarfs are too dim or distant for us to see with our naked eyes, 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 be otherwise 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 clip 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 as a preprint on arXiv.
"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 vary 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 that are 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 that the pollution would 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.
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 the Earth, they follow the Earth's magnetic field lines and then they make a spot on the 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 to a couple 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 this is a difficult theory to back up with current techniques. That said, there are two white dwarfs on record that appear to support it.
The study also raises another issue that's tied to how much planetary material needs to be out there, 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."
The research was supported by the U-M Rackham Merit Fellowship, the Fannie and John Hertz Foundation and the McCray Postdoctoral Fellowship at the University of Colorado Boulder.
