15:30 〜 15:45
[PCG19-17] A possible correlation between the metal pollution of white dwarfs and the "dirtiness" of their dust disks
キーワード：白色矮星の重元素汚染、惑星組成、赤外スペクトル
A quarter to half of white dwarfs have metals in their atmospheres (e.g., Zuckerman et al. 2010). They are thought to originate from minor planets that orbit white dwarfs, enabling us to probe the solid composition of planetary bodies beyond the solar system. Dust disks observed around metal-polluted white dwarfs could provide additional compositional insights through thermal emission spectra (Reach et al. 2009).
Nevertheless, it remains unclear whether the dust disk is the origin of the metals in white dwarfs. If this is the case, a correlation should exist between the elemental composition of white dwarf atmospheres and the dust composition of the disks. In this study, we investigate a potential correlation between the two. By comparing observed infrared disk spectra around white dwarfs (Jura et al. 2009) with those calculated from a disk thermal emission model (Chiang & Goldreich 1997), we estimate the abundance of materials with the conductivity orders of magnitude higher than that of insulators (e.g., silicates) in the disk dust. We then compare it with the measured stellar metallicity.
If the materials with much higher conductivity are iron-bearing species such as metallic iron, FeO, and Fe3O4, we, for the first time, discover a correlation between the Mg:Fe:Si ratio in white dwarf atmospheres and the abundance of the metallic iron or the iron oxides in the disk dust. Because the materials with much higher conductivity than insulators increase the dust opacity at ~ 5 µm, the emission from an iron-bearing dust disk is larger than that from a disk composed solely of silicate dust. This provides direct evidence of photospheric pollution through disk accretion. Additionally, we find iron-rich dust in several disks, potentially originating from the cores of differentiated bodies.
Nevertheless, it remains unclear whether the dust disk is the origin of the metals in white dwarfs. If this is the case, a correlation should exist between the elemental composition of white dwarf atmospheres and the dust composition of the disks. In this study, we investigate a potential correlation between the two. By comparing observed infrared disk spectra around white dwarfs (Jura et al. 2009) with those calculated from a disk thermal emission model (Chiang & Goldreich 1997), we estimate the abundance of materials with the conductivity orders of magnitude higher than that of insulators (e.g., silicates) in the disk dust. We then compare it with the measured stellar metallicity.
If the materials with much higher conductivity are iron-bearing species such as metallic iron, FeO, and Fe3O4, we, for the first time, discover a correlation between the Mg:Fe:Si ratio in white dwarf atmospheres and the abundance of the metallic iron or the iron oxides in the disk dust. Because the materials with much higher conductivity than insulators increase the dust opacity at ~ 5 µm, the emission from an iron-bearing dust disk is larger than that from a disk composed solely of silicate dust. This provides direct evidence of photospheric pollution through disk accretion. Additionally, we find iron-rich dust in several disks, potentially originating from the cores of differentiated bodies.