3:00 PM - 3:15 PM
[PPS06-06] Constructing a model of metal accretion disks around white dwarf stars: toward understanding the evolution of remnant planetary systems
Keywords:White dwarf stars, Debris disks, Exoplanets
White dwarfs are the final evolutionary state of stars with a mass less than ~8 Msun. 1/2-1/4 of all WDs exhibit metal pollution in their atmospheres (Zuckerman et al. 2010), and some of them show infrared excess from their circumstellar debris disks (Rocchetto et al. 2015). These are thought to be originated from the remnant planetary systems around WDs which evolve with their central stars. The surviving asteroids/minor planets are gravitationally scattered onto star-grazing orbits, and the subsequent tidal disruption is likely to produce compact accretion disks. Based on this scenario, an accretion disk model is essential to link a rapidly growing number of these observations around WDs to surviving planetary systems as well as their evolution.
In this study, we construct a 1-dimensional WD disk model that consistently solves key processes in such disks: advection, diffusion, sublimation, and condensation. We firstly investigated the time evolution of accretion disks composed of silicate particles and their vapor. For the particulate disk, Poynting-Robertson (PR) drag is thought to drive the metal accretion. On the other hand, observations have shown that WDs accompanied by debris disks tend to have accretion rates orders of magnitude higher than that can be explained by PR drag alone (~ 1e8 g/s). Metzger et al. (2012) demonstrated that if the aerodynamic drag due to vapor produced by the sublimation of silicate particles is effective, the accretion rate could reach such measured values. However, they neglected the back-reaction and diffusion of particles in the gas and condensation of silicate vapor. As a result of our calculation including these effects, we found that the accretion rate enhancement proposed by Metzger et al. (2012) does not occur because the vapor density determined by the saturating vapor pressure outside the sublimation line is so low that it cannot affect the particle motion. Furthermore, near the silicate sublimation line, silicate vapor advects/diffuses outward and then recondenses into the solid particles. This causes a pile-up of the particles just outside the sublimation line, resulting in a lower accretion rate than even that predicted by PR drag alone. Finally, we will discuss the possibility that the disk with vapor produced by solid components with lower sublimation points than silicates (e.g., water ice) could reproduce the observational high accretion rates.
In this study, we construct a 1-dimensional WD disk model that consistently solves key processes in such disks: advection, diffusion, sublimation, and condensation. We firstly investigated the time evolution of accretion disks composed of silicate particles and their vapor. For the particulate disk, Poynting-Robertson (PR) drag is thought to drive the metal accretion. On the other hand, observations have shown that WDs accompanied by debris disks tend to have accretion rates orders of magnitude higher than that can be explained by PR drag alone (~ 1e8 g/s). Metzger et al. (2012) demonstrated that if the aerodynamic drag due to vapor produced by the sublimation of silicate particles is effective, the accretion rate could reach such measured values. However, they neglected the back-reaction and diffusion of particles in the gas and condensation of silicate vapor. As a result of our calculation including these effects, we found that the accretion rate enhancement proposed by Metzger et al. (2012) does not occur because the vapor density determined by the saturating vapor pressure outside the sublimation line is so low that it cannot affect the particle motion. Furthermore, near the silicate sublimation line, silicate vapor advects/diffuses outward and then recondenses into the solid particles. This causes a pile-up of the particles just outside the sublimation line, resulting in a lower accretion rate than even that predicted by PR drag alone. Finally, we will discuss the possibility that the disk with vapor produced by solid components with lower sublimation points than silicates (e.g., water ice) could reproduce the observational high accretion rates.