WD 0816−310

Lore & Background

WD 0816−310 was first identified as a possible white dwarf in 2005 from data of the Digitized Sky Survey. It was confirmed as a white dwarf in 2008 with spectroscopic data from CTIO, and the same team found that the white dwarf is polluted with calcium, magnesium and iron. In 2019 a variable magnetic field was discovered thanks to Zeeman splitting, using archived spectropolarimetric data from FORS1 at the Very Large Telescope (VLT). In 2021 the white dwarf was studied in detail with the 4 m telescope at CTIO and with the VLT (FORS1 and X-shooter), detecting sodium, magnesium, calcium, chromium, manganese, iron and nickel in its atmosphere. The atmosphere is enriched in magnesium relative to other elements, which is predicted for old stellar systems. Hydrogen was also found in this otherwise helium-dominated atmosphere, possibly explained by pollution from an asteroid containing water ice. Researchers found that the abundance of metals changed between two spectra taken 10 years apart, suggesting spots enriched in metals are present on the surface, controlled by the magnetic field. In 2024 this was confirmed with circular spectropolarimetric observations with FORS2 on the VLT, measuring a dipolar field strength at the pole of about 140 Kilogauss. Around 310,000 years ago WD 0816−310 accreted a Vesta-sized object with a composition similar to chondritic meteorites. The observations showed that the variation in metal line strength and magnetic field intensity are synchronized, seen as evidence that the magnetic field determines the local density of metals on the surface. These patches are likely present near one of the magnetic poles. The material from the accreted asteroid first formed a disk around the white dwarf; closer to the white dwarf the dusty material sublimated into a metal-gas. The white dwarf ionizes at least part of the gas, and these ions follow the magnetic field, spiraling around local field lines due to the Lorentz force. On their way to the poles, the ions collide with neutral atoms in the gas disk, ionizing them and leading to substantial ionization of the disk.

Reader's Guide

WD 0816−310 is significant as the first magnetic white dwarf where a metal 'scar'—a localized patch of accreted planetary material—was confirmed to be synchronized with the magnetic field. The 2024 observations demonstrated that variations in metal line strength and magnetic field intensity are synchronized, providing direct evidence that the magnetic field controls the distribution of metals on the surface. This supports the model in which ions from a disrupted planetary body are channeled along magnetic field lines to the poles, creating enriched spots. The object also illustrates the process of planetary debris accretion: a Vesta-sized object with chondritic composition was accreted around 310,000 years ago, forming a disk that sublimated into metal-gas, which was then ionized and guided by the magnetic field. The detection of hydrogen suggests the accreted body may have contained water ice. A 2024 study that discovered a second metal scar around WD 2138−332 suggests that such scars are common around magnetic white dwarfs with metal pollution, making WD 0816−310 a prototype for understanding how magnetic fields shape the surface composition of white dwarfs and record the history of planetary system evolution.

Did You Know?

• WD 0816−310 has a magnetic field strength at the pole of about 140 Kilogauss.
• The white dwarf accreted a Vesta-sized object around 310,000 years ago.
• Hydrogen detected in its helium-dominated atmosphere may come from an asteroid containing water ice.
• The metal scar was confirmed in 2024 using circular spectropolarimetric observations with FORS2 on the VLT.