Formation, diffusion and accreting pollution of DB white dwarfs

TL;DR

This study models the evolution of metal pollution in DB white dwarfs, showing that changes in accretion rates during cooling explain observed abundance sequences.

Contribution

It introduces a stellar evolution model that accounts for variable accretion rates to explain metal pollution patterns in DB white dwarfs.

Findings

Mass-accretion rate and temperature determine heavy element abundances.

Constant accretion rate models cannot explain observed sequences.

Variable accretion rates aligned with WD cooling explain observations.

Abstract

Over 1500 DBZ or DZ white dwarfs (WDs) have been observed so far, and polluted atmospheres with metal elements have been found among these WDs. The surface heavy element abundances of known DBZ or DZ WDs show an evolutionary sequence. By using Modules for Experiments in Stellar Evolution, we create DB WDs, and simulate the element diffusion due to high gravitational fields and the metal-rich material accretion coming from the planet disrupted by the WD. In our models, the input parameters ($\alpha_{\rm MLT}$, $\alpha_{\rm th}$ and $Z$) have very weak effect on DB WD structures including interior temperatures, chemical profiles and convective zones.The mass-accretion rate and the effective temperature of DB WDs determine the abundances of heavy elements. The evolutionary sequence of Ca element for about 1500 observed DB or DBZ WDs cannot be explained by the model with a constant mass-accretion rate, but is consistent well with the model in which the mass-accretion rate decreases by one power law when $T_{\rm eff}>10$ kK and slightly increases by another power law when $T_{\rm eff}<10$ kK. The observed DB WD evolutionary sequence of heavy element abundances originates from WD cooling and the change of mass-accretion rate.