# He white dwarfs with large H contamination: Convective mixing or accretion?

## TL;DR

This study investigates the origin of hydrogen in helium-rich white dwarfs with heavy elements, concluding that accretion from debris disks is the most plausible source rather than convective mixing.

## Contribution

It provides full evolutionary models for He-H-Z white dwarfs and demonstrates that hydrogen accretion from debris disks explains observed atmospheric compositions.

## Key findings

- Convective mixing cannot account for hydrogen levels in these white dwarfs.
- Hydrogen accretion from water-bearing debris disks is the most likely source.
- Models match observed atmospheric compositions of He-H-Z white dwarfs.

## Abstract

White dwarfs are compact objects with atmospheres containing mainly light elements, hydrogen or helium. Because of their surface high gravitational field, heavy elements diffuse downwards in a very short timescale compared to the evolutionary timescale, leaving the lightest ones on the top of the envelope. This results in the main classification of white dwarfs as hydrogen rich or helium rich. But many helium rich white dwarfs show also the presence of hydrogen traces in their atmosphere, whose origin is still unsettled. Here we study, by means of full evolutionary calculations, the case for a representative model of the "He-H-Z" white dwarfs, a sub-group of helium rich white dwarfs showing both heavy elements and a large amount of hydrogen in their atmosphere. We find it impossible to explain its hydrogen atmospheric content by the convective mixing of a primordial hydrogen present in the star. We conclude that the most likely explanation is the accretion of hydrogen rich material, presumably water-bearing, coming from a debris disk.