13:45 〜 15:15
[PEM11-P01] White dwarf pollution by rocky/icy planetary bodies: modeling the evolution of silicate/volatile accretion disks
キーワード：白色矮星の重元素汚染、惑星組成、 デブリ円盤
A growing number of debris disks have been detected around metal-polluted white dwarfs. They are thought to originate from tidally disrupted planetary bodies and are responsible for metal accretion onto host WDs. Photospheric observations have shown that (1) WDs with disks would typically have accretion rates higher than that induced by Poynting-Robertson (PR) drag (Farihi et al. 2009), and that (2) the vast majority of them would be polluted by materials with refractory-rich rocky composition (Jura & Young 2014). To explain (1) and (2), Metzger et al. (2012) developed the first accretion disk model that formulates the interaction between silicate particles and silicate vapor and proposed runaway accretion of silicate particles due to gas drag by the increasing silicate vapor produced by the sublimation of the particles. However, the effect of re-condensation of the silicate vapor remained an unsolved issue.
In this study, we revisit this problem by one-dimensional advection/diffusion simulation that consistently incorporates silicate sublimation/condensation and back-reaction to particle drift due to gas drag in the solid-rich disk. We find that because silicate vapor density in the region overlapping the solid particles can exist only up to the saturating vapor pressure, no runaway accretion occurs if the re-condensation is taken into account. This always limits the accretion rate from mono-compositional silicate disks to the PR drag flux in the equilibrium state.
As dynamical arguments allow the supply of solid bodies from a wide range of orbital radii (Bonsor et al. 2011; Li et al. 2022), volatile gas would be possibly provided by the infall of ice-bearing bodies. Accordingly, we perform additional simulations that couple the volatile gas (e.g. water vapor). Because the volatile gas does not condense in the region where the silicate particle disk is distributed, we demonstrate that it enhances the silicate accretion to rates larger than PR drag flux through gas drag. The refractory-rich accretion is simultaneously reproduced when the initial volatile fraction of the disk is less than ~10 wt% because of the suppression of volatile accretion due to the efficient back-reaction of solid to gas. The C-type asteroid analogs like Ceres would be a plausible origin of such disks with a small but non-negligible fraction of volatiles.
In this study, we revisit this problem by one-dimensional advection/diffusion simulation that consistently incorporates silicate sublimation/condensation and back-reaction to particle drift due to gas drag in the solid-rich disk. We find that because silicate vapor density in the region overlapping the solid particles can exist only up to the saturating vapor pressure, no runaway accretion occurs if the re-condensation is taken into account. This always limits the accretion rate from mono-compositional silicate disks to the PR drag flux in the equilibrium state.
As dynamical arguments allow the supply of solid bodies from a wide range of orbital radii (Bonsor et al. 2011; Li et al. 2022), volatile gas would be possibly provided by the infall of ice-bearing bodies. Accordingly, we perform additional simulations that couple the volatile gas (e.g. water vapor). Because the volatile gas does not condense in the region where the silicate particle disk is distributed, we demonstrate that it enhances the silicate accretion to rates larger than PR drag flux through gas drag. The refractory-rich accretion is simultaneously reproduced when the initial volatile fraction of the disk is less than ~10 wt% because of the suppression of volatile accretion due to the efficient back-reaction of solid to gas. The C-type asteroid analogs like Ceres would be a plausible origin of such disks with a small but non-negligible fraction of volatiles.