# Convective Overshoot and Macroscopic Diffusion in Pure-Hydrogen Atmosphere White Dwarfs

## TL;DR

This paper uses advanced 3D simulations to study how convective overshoot affects macroscopic diffusion in pure-hydrogen white dwarf atmospheres, revealing significant increases in mixed region mass and settling times.

## Contribution

It introduces a new grid of 3D white dwarf models to quantify macroscopic diffusion coefficients caused by convective overshoot, improving understanding of white dwarf atmospheric mixing.

## Key findings

- Mass of fully mixed region can increase by up to 2.5 orders of magnitude.
- Inferred accretion rates increase by about an order of magnitude.
- Settling times can increase by up to 2 orders of magnitude.

## Abstract

We present a theoretical description of macroscopic diffusion caused by convective overshoot in pure-hydrogen DA white dwarfs using three-dimensional (3D), closed-bottom, radiation hydrodynamics CO$^5$BOLD simulations. We rely on a new grid of deep 3D white dwarf models in the temperature range 11400 K $\leq T_{\mathrm{eff}} \leq$ 18000 K where tracer particles and a tracer density are used to derive macroscopic diffusion coefficients driven by convective overshoot. These diffusion coefficients are compared to microscopic diffusion coefficients from one-dimensional structures. We find that the mass of the fully mixed region is likely to increase by up to 2.5 orders of magnitude while inferred accretion rates increase by a more moderate order of magnitude. We present evidence that an increase in settling time of up to 2 orders of magnitude is to be expected which is of significance for time-variability studies of polluted white dwarfs. Our grid also provides the most robust constraint on the onset of convective instabilities in DA white dwarfs to be in the effective temperature range from 18000 to 18250 K.