Metal accretion scars may be common on magnetic, polluted white dwarfs

TL;DR

This study suggests that metal accretion scars are likely common on magnetic, polluted white dwarfs, based on observations of variability in metal lines and magnetic fields that imply localized metal concentrations near magnetic poles.

Contribution

It reports the discovery of a second magnetic, metal-polluted white dwarf with correlated variability in magnetic field and metal lines, indicating localized metal accretion regions.

Findings

Metal line strengths vary in phase with magnetic field measurements.

Variability suggests metal concentration around magnetic poles.

Magnetic accretion scars may be common on such white dwarfs.

Abstract

More than 30% of white dwarfs exhibit atmospheric metals, which are understood to be from recent or ongoing accretion of circumstellar debris. In cool white dwarfs, surface motions should rapidly homogenise photospheric abundances, and the accreted heavy elements should diffuse inward on a timescale much longer than that for surface mixing. The recent discovery of a metal scar on WD0816-310 implies its magnetic field has impeded surface mixing of metals near the visible magnetic pole. Here, we report the discovery of a second magnetic, metal-polluted white dwarf, WD2138-332, which exhibits periodic variability in longitudinal field, metal line strength, and broadband photometry. All three variable quantities have the same period, and show remarkable correlations: the published light curves have a brightness minimum exactly when the longitudinal field and line strength have a maximum, and a maximum when the longitudinal field and line strength have a minimum. The simplest interpretation of the line strength variability is that there is an enhanced metal concentration around one pole of the magnetic field; however, the variable line-blanketing cannot account for the observed multi-band light curves. More theoretical work is required to understand the efficiency of horizontal mixing of the accreted metal atoms, and the origin of photometric variability. Because both magnetic, metal-polluted white dwarfs that have been monitored to date show that metal line strengths vary in phase with the longitudinal field, we suggest that metal scars around magnetic poles may be a common feature of metal-polluted white dwarfs.