Accretion and diffusion in the DAZ white dwarf GALEX J1931+0117

TL;DR

This study analyzes high-quality spectra of the DAZ white dwarf GALEX J1931+0117, revealing detailed elemental abundances, pressure effects on spectral lines, and accretion rates, suggesting accretion from a debris disc.

Contribution

It provides the first detailed spectral analysis of GALEX J1931+0117, including pressure effects, elemental abundances, and accretion modeling, indicating accretion from a debris disc.

Findings

Silicon lines show pressure effects consistent with laboratory data.

Elemental abundances exceed or are below solar levels, indicating selective accretion.

Accretion rates vary significantly among elements, with oxygen being dominant.

Abstract

We present an analysis of high-dispersion and high signal-to-noise ratio spectra of the DAZ white dwarf GALEX J1931+0117. The spectra obtained with the VLT-Kueyen/UV-Visual Echelle Spectrograph show several well-resolved Si II spectral lines enabling a study of pressure effects on line profiles. We observed large Stark shifts in silicon lines in agreement with laboratory measurements. A model atmosphere analysis shows that the magnesium, silicon and iron abundances exceed solar abundances, while the oxygen and calcium abundances are below solar. Also, we compared the observed line profiles to synthetic spectra computed with variable accretion rates and vertical abundance distributions assuming diffusion steady-state. The inferred accretion rates vary from dM/dt=2x10^6 for calcium to 2x10^9 g/s for oxygen and indicate that the accretion flow is dominated by oxygen, silicon and iron while being deficient in carbon, magnesium and calcium. The lack of radial velocity variations between two measurement epochs suggests that GALEX J1931+0117 is probably not in a close binary and that the source of the accreted material resides in a debris disc.