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Using the James Webb Space Telescope (JWST), astronomers have performed infrared observations of a planetary debris disk around a nearby white dwarf known as GD 362. Results of the new observations, October 8 on the arXiv preprint server, yield important insights into the chemical composition of this disk.

White dwarfs (WDs) are stellar cores left behind after a star has exhausted its nuclear fuel. Due to their high gravity, they are known to have atmospheres of either pure hydrogen or pure helium.

However, there exists a small fraction of WDs that shows traces of heavier elements, and they are believed to be accreting planetary material. Studies of this material around WDs, which often forms dust disks, is essential to improving our knowledge of how planets form and evolve.

GD 362, also known as WD 1729+371, is a white dwarf located some 182.9 light years away. It has a radius of about 8,790 kilometers, a mass of approximately 0.57 solar masses, and its effective temperature is estimated to be 9,825 K.

Previous observations of GD 362 show that it is one of the most heavily polluted white dwarfs, and showcases an exceptionally strong infrared excess. It turned out that it possesses a helium-dominated atmosphere with a high metal abundance, and also an anomalously large mass of hydrogen. Moreover, a dust disk was identified, located within 140 to 1,400 stellar radii of GD 362.

Recently, a team of astronomers led by William T. Reach of the Space Science Institute in Boulder, Colorado, employed JWST's Near Infrared Spectrograph (NIRSpec) and Mid-Infrared Instrument (MIRI) to take a closer look at the disk around GD 362, hoping to shed more light on its chemical composition.

The spectral resolution and sensitivity of the two instruments allowed Reach's team to measure the composition of solid planetary material around GD 362. The mid-infrared spectrum was found to be dominated by an exceptionally strong 9–11 µm silicate feature, three times brighter than its underlying continuum, which extends to at least 2 µm, and requires hot debris (with a temperature of about 950 K) close to the white dwarf.

The results indicate that the disk around GD 362 contains a mix of amorphous and crystalline olivines and pyroxenes plus amorphous carbon. It was found that the elemental abundances of carbon, oxygen, magnesium, aluminum and iron are within a factor of two, relative to silicon.

It was noted that no evidence for water in the spectrum, nor were other hydrogen-bearing species found. This suggests the dust in the disk is drier and lower in hydrogen than in chondritic meteorites.

"Overall, the results indicate that GD 362 is surrounded by a disk with solids having elemental abundances approximately matching those seen in the atmosphere of the white dwarf, supporting the connection between disk and atmosphere arising from accretion of planetary material," the authors of the paper conclude.

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