Global modeling of radiatively driven accretion of metals from compact debris disks onto the white dwarfs

TL;DR

This paper models the global evolution of debris disks around white dwarfs under Poynting-Robertson drag, revealing how disk properties influence metal accretion rates and disk longevity, with implications for understanding white dwarf pollution.

Contribution

It extends local analyses to a global model, demonstrating how initial disk structure affects accretion rates and disk evolution, especially for optically thick disks.

Findings

Massive optically thick disks produce high metal accretion rates.

Disk lifetime can reach several million years.

Optically thin disks have lower accretion rates and shorter evolution timescales.

Abstract

Recent infrared observations have revealed presence of compact (radii < R_Sun) debris disks around more than a dozen of metal-rich white dwarfs (WD), likely produced by tidal disruption of asteroids. Accretion of high-Z material from these disks may account for the metal contamination of these WDs. It was previously shown using local calculations that the Poynting-Robertson (PR) drag acting on the dense, optically thick disk naturally drives metal accretion onto the WD at the typical rate \dot M_{PR} \approx 10^8 g/s. Here we extend this local analysis by exploring global evolution of the debris disk under the action of the PR drag for a variety of assumptions about the disk properties. We find that massive disks (mass > 10^{20} g), which are optically thick to incident stellar radiation inevitably give rise to metal accretion at rates \dot M > 0.2\dot M_{PR}. The magnitude of \dot M and its time evolution are determined predominantly by the initial pattern of the radial distribution of the debris (i.e. ring-like vs. disk-like) but not by the total mass of the disk. The latter determines only the disk lifetime, which can be several Myr or longer. Evolution of an optically thick disk generically results in the development of a sharp outer edge of the disk. We also find that the low mass (< 10^{20} g), optically thin disks exhibit \dot M << \dot M_{PR} and evolve on characteristic timescale \sim 10^5-10^6 yr, independent of their total mass.