The prototype double-faced white dwarf has a thin hydrogen layer across its entire surface

TL;DR

This study reveals a white dwarf with a thin, inhomogeneous hydrogen layer across its surface, explaining spectral variations through stratified atmosphere models and suggesting complex surface and internal processes.

Contribution

The paper introduces stratified atmosphere models to successfully interpret spectral variations in a double-faced white dwarf, advancing understanding of surface composition inhomogeneities.

Findings

Excellent spectral fits with stratified models

Hydrogen layer varies from optically thick to thin

Implications for magnetic fields and internal hydrogen distribution

Abstract

Some white dwarfs undergo significant changes in atmospheric composition owing to the diffusion and mixing of residual hydrogen in a helium-rich envelope. Of particular interest are a few objects exhibiting hydrogen and helium line variations modulated by rotation, revealing surface composition inhomogeneities. Recently, the hot ultramassive white dwarf ZTF J203349.80+322901.1 emerged as the most extreme such specimen, with hydrogen and helium lines successively appearing and vanishing in anti-phase, suggesting a peculiar double-faced configuration. However, standard atmosphere models fail to reproduce the observed spectrum at all rotation phases, hampering further interpretation. Here, we perform a new analysis of ZTF J203349.80+322901.1 using stratified atmosphere models, where hydrogen floats above helium, and obtain excellent fits to the phase-resolved spectra. Our results imply that an extremely thin hydrogen layer covers the entire surface but varies from optically thick to optically thin across the surface, thus producing the observed spectral variations. We present new envelope models indicating that the hydrogen layer arises from a delicate interplay between diffusion and convection. We discuss possible explanations for the surface layer asymmetry, including an asymmetric magnetic field and a non-uniform internal hydrogen distribution. Finally, we highlight implications for expanding and understanding the emerging class of inhomogeneous white dwarfs.