Metal âscarâ found on dying star ingesting planets and asteroids
26 February 2024
The unique signature of a star ingesting its surrounding planets and asteroids â a metal scar imprinted on the surface of a white dwarf star â has been found for the first time by a team including MyAV·¶ researchers.
White dwarfs are glowing embers of stars that have burned through all their hydrogen fuel. Nearly all stars, including the Sun, will eventually become white dwarfs.
For the new study, published inÌęThe Astrophysical Journal Letters, the research team usedÌęthe European Southern Observatoryâs Very Large Telescope (ESOâs VLT) in Chile toÌęobserve a concentration of metals imprinted on the surfaceÌęof the white dwarf WD 0816-310, an Earth-sized remnant of a star somewhat more massive than the Sun.
They concluded that the starâs magnetic field funnelled metals on to its surface, creating the scar, a feature never seen before on a white dwarf star.
Lead author Dr Stefano Bagnulo (Armagh Observatory and Planetarium in Northern Ireland, andÌęHonorary Associate Professor atÌęMullard Space Science Laboratory at MyAV·¶), said: âIt is well known that some white dwarfs are cannibalising pieces of their planetary systems. Now we have discovered that the starâs magnetic field plays a key role in this process, resulting in a scar on the white dwarfâs surface.â
Co-author Professor Jay Farihi (MyAV·¶ Physics & Astronomy): âOur analysis demonstrates that this metal scar has been present on the star for much longer than predicted by any theory, and that the material is highly Earth-like in its composition.Ìę Altogether, we can deduce that theÌęmetals originate from a planetary fragment as large as or possibly larger than Vesta; around 500 kilometres across, it is the second-largest asteroid in the Solar System, and often referred to as a planetary embryo or proto-planet."
The observations provided clues to how the star got its metal scar. The team noticed that the strength of the metal detection changed as the star rotated, suggesting that the metals were concentrated on a specific area on the white dwarfâs surface, rather than smoothly spread across it. They also found that these changes were synchronised with changes in the white dwarfâs magnetic field, indicating that this metal scar was located on one of its magnetic poles. Together, these clues indicated that the magnetic field funnelled metals on to the star.
Previously, astronomers have observed numerous white dwarfs polluted by metals that were scattered across the surface of the star. These are known to originate from disrupted planets or asteroids that veer too close to the star, following star-grazing orbits similar to those of comets in our Solar System.
To reach these conclusions, the team used a âSwiss-army knifeâ instrument on theÌęVLTÌęcalledÌęFORS2, which allowed them to detect the metal scar and connect it to the starâs magnetic field.
Dr Bagnulo said: âESO has the unique combination of capabilities needed to observe faint objects such as white dwarfs, and sensitively measure stellar magnetic fields.â
In their study, the team also relied on archival data from the VLTâsÌęX-shooterÌęinstrument to confirm their findings.
Harnessing the power of observations like these, astronomers can reveal the bulk composition of exoplanets, planets orbiting other stars outside the Solar System. This unique study also shows how planetary systems can remain dynamically active, even after 'death'.
Links
- Professor Jay Farihiâs academic profile
- Mullard Space Science Laboratory at MyAV·¶
- MyAV·¶ Physics & Astronomy
- MyAV·¶ Mathematical & Physical Sciences
Image and video
- This artistâs impression shows the magnetic white dwarf WD 0816-310, where astronomers have found a scar imprinted on its surface as a result of having ingested planetary debris.ÌęWhen objects like planets or asteroids approach the white dwarf they get disrupted, forming a debris disc around the dead star. Some of this material can be devoured by the dwarf, leaving traces of certain chemical elements on its surface.ÌęUsing ESOâs Very Large Telescope, astronomers found that the signature of these chemical elements changed periodically as the star rotated, as did the magnetic field. This indicates that the magnetic fields funnelled these elements onto the star, concentrating them at the magnetic poles and forming the scar seen here.ÌęCredit:ÌęESO/L. Calçada
Media contact
Mark Greaves
m.greaves [at] ucl.ac.uk