Runaway accretion of metals from compact debris disks onto white dwarfs

TL;DR

This paper presents a model explaining how white dwarfs can rapidly accrete metals from debris disks through interaction with a metallic gas disk, accounting for observed high accretion rates.

Contribution

The authors propose a novel runaway accretion mechanism involving gas-disk interactions that explains high metal accretion rates onto white dwarfs.

Findings

Runaway accretion can reach rates up to 10^{11} g/s.

The process destroys debris disks within approximately 10^5 years.

Model aligns with observed metal accretion rates in white dwarfs.

Abstract

It was recently proposed that metal-rich white dwarfs (WDs) accrete their metals from compact debris disks found to exist around more than a dozen of them. At the same time, elemental abundances measured in atmospheres of some WDs imply vigorous metal accretion at rates up to $10^{11}$ g/s, far in excess of what can be supplied solely by Poynting-Robertson drag acting on such debris disks. To explain this observation we propose a model, in which rapid transport of metals from the disk onto the WD naturally results from interaction between this particulate disk and spatially coexisting disk of metallic gas. The latter is fed by evaporation of debris particles at the sublimation radius located at several tens of WD radii. Because of pressure support gaseous disk orbits WD slower than particulate disk. Resultant azimuthal drift between them at speed ~1 m/s causes aerodynamic drag on the disk of solids and drives inward migration of its constituent particles. Upon reaching the sublimation radius particles evaporate, enhancing the density of metallic gaseous disk and leading to positive feedback. Under favorable circumstances (low viscosity in the disk of metallic gas and efficient aerodynamic coupling between the disks) system evolves in a runaway fashion, destroying debris disk on time scale of $\sim 10^5$ yr, and giving rise to high metal accretion rates up to $10^{10}-10^{11}$ g/s, in agreement with observations.