An article published in “The Astrophysical Journal Letters” reports the results of a study of the white dwarf cataloged as WD 0816-310 and of what was defined as a scar on its surface, left by the materials of a large asteroid that got swallowed. A team of researchers used ESO’s VLT (Very Large Telescope) in Chile to obtain the information necessary to identify the metallic elements that form that scar and its concentration in a specific area, which is one of its magnetic poles. This result offers new insights into the evolution of planetary systems after the death of their star.
When the core of a small to medium star runs out of hydrogen, it begins an agony that ends with its collapse into a white dwarf. Its mass may be greater than the Sun’s but without the energy of nuclear fusion, gravity causes it to compact into a volume similar to the Earth’s. Planets have been discovered orbiting the remnants of dead stars but the dynamics of these planetary systems are still poorly understood, so the study of the white dwarf WD 0816-310 is important.
Among the processes known in planetary systems existing around the remnant of dead stars is the possible destruction of bodies whose orbit was destabilized during the star’s death throes. Circumstellar debris was observed, as well as traces of a sort of cannibalism in which the dead star devours one or more of these bodies.
Knowledge of these destructive processes is limited and until recently astronomers thought that debris from swallowed bodies was distributed rather homogeneously on the surface of a white dwarf. However, this new study showed a very different situation.
Just over 60 light-years away from Earth, the white dwarf WD 0816-310 showed traces of the presence of various heavy elements such as carbon, magnesium, and iron. A few years ago, a variable magnetic field was detected. WD 0816-310 was the subject of various studies and the comparison between the data collected over the years indicated a change in the abundance of metals detected, leading to the hypothesis of a link with its magnetic field. This new study has shed light on this link.
The FORS 2 (FOcal Reducer and low dispersion Spectrograph 2) instrument mounted on the VLT was used to detect the metal scar on the surface of the white dwarf WD 0816-310 and connect it with its magnetic field. Archival data from another VLT instrument, X-shooter, helped confirm the researchers’ findings.
The data collected allowed to find evidence that the metals detected on the surface of the white dwarf WD 0816-310 are localized on one of its magnetic poles, a distribution linked to the magnetic field. The metals come from a large asteroid or dwarf planet that was compared to the giant asteroid Vesta. In the reconstruction made by the researchers, the object was destroyed when it approached the white dwarf and its metals were ionized by the magnetic field, with the consequence that they were funneled towards one of its magnetic poles.
A few years ago, NASA’s Dawn space probe visited Vesta to collect data that led astronomers to conclude that it’s a protoplanet. The presence of Jupiter stopped the growth of Vesta, which is one of the many bodies in the asteroid belt between Mars and Jupiter. In the system of WD 0816-310, it’s possible that there was a gas giant that had the same effect on the body that was later swallowed by the white dwarf.
These conclusions were possible thanks to the availability of data collected over more than a decade complemented by high-quality data collected thanks to powerful and sensitive instruments such as those mounted on the VLT. Many studies are based on single observation campaigns but certain processes require multiple observations, sometimes over the course of years, to see their effects and understand their mechanisms. That’s what the authors of the study on the white dwarf WD 0816-310 did and that study also offers useful ideas for studying other planetary systems even after their star died.
