A Possible Indication of Metallic Iron in White Dwarf Dusty Disks from their "Dirtiness"

TL;DR

This study tests whether metallic iron mixed into silicate dust can account for the near-infrared excess observed in white-dwarf debris disks. It combines Mie-based opacities for Fe-bearing silicates with a Bruggeman mixing framework and a two-layer Chiang–Goldreich disk model to generate synthetic spectra, which are then fitted to Spitzer data for G29-38 and GD56. The results indicate that Fe-bearing dust with a metal-to-silicate mixing ratio of about $y \,\approx\,1$ provides a better match to G29-38 and is broadly consistent with its atmospheric iron abundance, though Fe-rich silicates without metallic iron are not definitively excluded. For GD56, iron-bearing dust is favored but large abundance uncertainties limit a precise comparison; the authors anticipate JWST surveys to enable robust statistical analysis of iron-dust correlations in polluted white dwarfs.

Abstract

Polluted white dwarfs provide unique constraints on the elemental compositions of planetary bodies. The tidal disruption of accreting bodies is thought to form circumstellar dusty disks, whose emission spectra could offer additional insights into the mineral phases of the accreted solid material. Silicates are detected in the mid-infrared spectra of several disks, but do not fully account for the near-infrared excess in the disks' spectra. Conductive materials, such as metallic iron, are potential sources of near-infrared emissivity. We investigate the role of metallic iron within silicate dust in the observed spectra of the white dwarfs G29-38 and GD56. Using thermal emission spectra calculations, we analyze the abundance of metallic iron in the dust and the disk structure parameters that best fit the observed spectra. We find that metallic-iron-bearing dust enhances the near-infrared opacity, thereby providing a better fit to the G29-38 spectrum for various silicate compositions than metallic-iron-free dust. The best-fit metal-to-silicate mixing ratio is approximately unity, and for Mg-rich pyroxenes, this value is also consistent with G29-38's stellar atmospheric composition within 1-$σ$ observational uncertainties. Based on the spectral fitting and compositional consistency, Fe-rich silicates without metallic iron cannot be ruled out. The observed GD56 spectrum also favors iron-bearing dust. However, the large observational uncertainties of GD56's stellar elemental abundances hinder a precise comparison between the stellar and dust iron abundances. Upcoming high-precision JWST observations will provide a larger sample, enabling statistical analysis of the correlation between the iron abundances in the atmospheres and circumstellar dust of polluted white dwarfs.

Figures (4)

• Figure 1: Infrared spectra of the G29-38 and GD56 systems (left and right panels, respectively) used in this study. The Spitzer IRS spectra, binned Spitzer IRS spectra, and photometric data are plotted in green, gray, and black, respectively. The error bars represent 1$\sigma$ observational noises.
• Figure 2: Complex refractive indices $m=n+ik$ of MgFeSiO$_4$ (green) and metallic Fe (red). The solid and dotted lines show the real and imaginary parts, $n$ and $k$, respectively.
• Figure 3: Effective refractive indices of materials composed of MgFeSiO$_4$ and metallic iron with various number mixing ratio, $y$. The solid and dotted lines represent $n$ and $k$, respectively.
• Figure 4: Mass absorption opacity of 1 $\mu$m-sized dust grains composed of MgFeSiO$_4$ and metallic iron with various number mixing ratio, $y$.