Composition of planetary debris around the white dwarf GD 362

TL;DR

The study uses JWST spectroscopy (0.6–28 μm) to dissect the dusty debris disk around GD 362, a highly polluted white dwarf. It finds a strong 9–11 μm silicate feature requiring olivine and pyroxene, plus hot carbonaceous dust to explain near-infrared emission; the disk extends from about $140R_\star$ to $1400R_\star$ with a scale height exceeding $0.5R_\star$. Mineralogical decomposition shows silicate-dominated material (~83%) with near-CI chondrite-like abundances for C, O, Mg, Al, and Fe (Al enhanced, O depleted) and no evidence for water or phyllosilicates. The disk mass is at least $\sim2\times10^{19}$ g with plausible total solid mass up to $\sim10^{22}$ g when extrapolating to larger bodies, and there is no detected exoplanet companion down to $\sim25M_{\rm J}$; hydrogen in the atmosphere likely reflects past water-rich accretion rather than current disk content. Overall, the results link the WD’s atmospheric pollution to a dry, carbon-rich, mineralogically CI-like planetary debris disk surrounding GD 362.

Abstract

White dwarf stars with high abundances of heavy elements in their atmospheres and infrared excesses are believed to be accreting planetary material. GD 362 is one of the most heavily polluted white dwarfs and has an exceptionally strong mid-infrared excess, reprocessing 2.4% of the star's light into the mid-infrared. We present a high signal-to-noise, medium-resolution spectrum of GD 362 obtained with JWST, covering 0.6 to 17 microns, along with photometry out to 25.5 microns. The mid-infrared spectrum is dominated by an exceptionally strong 9 to 11 micron silicate feature, which can be explained by a combination of olivine and pyroxene silicate minerals. Grains such as carbon, hotter than silicates, are required to explain the near-infrared emission. The silicates and carbon reside in a disk from 140 to 1400 stellar radii, and the disk scale height is greater than half the stellar radius. The elemental abundances of the solid material, relative to Si, are within a factor of 2 of meteoritic (CI chondrites) for C, O, Mg, Al, and Fe, with Al elevated and O slightly depleted. A similar pattern is observed for the abundances of accreted material in the stellar photosphere. Hydrogen is an exception, because no significant H-bearing minerals or water were detected in the disk, despite a large H abundance in the photosphere.

Figures (7)

• Figure 1: Likelihood distribution for the initial mass of GD 362. The maximum likelihood initial-mass is $1.24\pm 0.13$$M_\odot$
• Figure 2: The JWST/NIRSPEC spectrum of GD 362, covering 0.6--5.3 $\mu$m. Dots show spectra from each exposure, with the color changing for each of the 5 nods along the slit. Insets show the spectra in the vicinity of the H$\alpha$ and P$\alpha$ photospheric lines of the white dwarf. The black curve shows the median of the 5 spectra. The black dots show the Spitzer/IRAC and Gaia photometry. For comparison, a white dwarf photosphere model, normalized to the Gaia $G_{\rm RP}$ flux, is shown in light purple.
• Figure 3: The JWST/MIRI spectrum of GD 362. The three colored curves show the spectra in MIRI/MRS channels 1, 2, and 3. The channel 4 spectrum is not shown because the star was not detected. The single-channel spike at 7.311 $\mu$m is likely an artifact. Black points show the photometry in five bands from Spitzer juraInfraredEmissionDusty2007, and purple dots show the photometry in three bands from JWST.
• Figure 4: Images of GD 362 in three JWST/MIRI filters at 18 $\mu$m ( top), 21 $\mu$m ( middle), and 25.5 $\mu$m ( bottom). For each filter, the direct images are shown on the ( left), and the images after subtraction of a point spread function centered on the star are in the ( right). In the PSF-subtracted images, a green circle denotes 100 au from the central star.
• Figure 5: Radiative transfer models for the spectrum of dust around GD 362. (a) The top panel shows an optically thin model (with a large scale height). (b) The bottom panel shows a moderately optically thick model (with a scale height 3.5 stellar radii at the inner edge). Grains in both models are composed of 'Draine' silicates, amorphous carbon, and forsterite. The dust distribution parameters are in Table \ref{['tab:params']}.