Investigating a Polluted White Dwarf

Roughly 25–50% of the white dwarfs in the solar neighborhood are “polluted,” showing signs of accretion of metal-rich planetary material. New JWST observations reveal the contents of the debris disk surrounding one of the most polluted white dwarfs in the solar neighborhood.

Heavy-Metal Pollution

White dwarfs are the remnants of low- to intermediate-mass stars that have ceased core fusion and evolved off the main sequence. The presence of metals in white dwarf atmospheres shows that it’s common for white dwarfs to accrete rocky materials from tidally disrupted bodies in their surroundings, allowing for a post-mortem investigation of the composition of rocky worlds in far-off planetary systems.

One of the most impressively metal-polluted white dwarfs known is GD 362, which is surrounded by a dusty debris disk and shows evidence for at least 16 accreted elements. GD 362 is a helium-atmosphere white dwarf, but previous observations have also shown that it has an unusually large amount of hydrogen in its atmosphere — a possible sign that the star has consumed one or more bodies containing water or ice.

Disk Investigation

Now, a team led by William Reach (Space Science Institute) has obtained JWST spectra of GD 362 to examine the material in its disk directly. Using both the Near-Infrared Camera (NIRCam) and the Mid-Infrared Instrument (MIRI), the team collected spectra of the white dwarf from 0.6 to 17 microns, with additional photometric measurements at 18, 21, and 25.5 microns. The JWST spectra show bright emission from the white dwarf at wavelengths shorter than 2.5 microns, with a clear signature of disk material at longer wavelengths, including a strong emission feature at 9–11 microns.

Reach’s team modeled the JWST spectra to constrain the mineralogy of the disk and its physical attributes. They found that the disk likely stretches from 140 to 1,400 times the white dwarf’s radius and contains both silicates and carbon, with the prominent emission feature pointing to olivine and pyroxene silicate minerals. The elements seen in the white dwarf’s atmosphere are generally similar to those observed in the disk, strengthening the connection between accretion from the disk and the metals present in the white dwarf’s spectrum.

Hydrogen Mismatch

The new JWST data furthered the investigation of GD 362’s atmospheric hydrogen and the possibility that it’s accreting water- or ice-rich material. Though water molecules have spectral features in the range studied, none of these features appeared in the JWST spectra of GD 362. This isn’t necessarily a surprise; the team showed that water molecules in the disk would quickly be split apart by GD 362’s high-energy radiation.

Curiously, though, Reach’s team found little evidence for hydrogen-rich material within the disk that could be the source of the white dwarf’s atmospheric hydrogen. Because accreted hydrogen would remain in the star’s atmosphere indefinitely, the presence of atmospheric hydrogen could be evidence that the white dwarf accreted water-rich material from its debris disk in the past, though the materials currently present in the disk are dry.

In addition to investigating GD 362’s disk, Reach and collaborators searched for planets orbiting the white dwarf. Though they found none, they were able to rule out the presence of companions with masses greater than 25 times the mass of Jupiter.

Citation

“Composition of Planetary Debris Around the White Dwarf GD 362,” William T. Reach et al 2025 ApJ 994 195. doi:10.3847/1538-4357/ae11a9