Astronomical research centre Armagh Observatory and Planetarium has played a leading role in the discovery of a unique signature of the process by which white dwarf stars cannibalise their planetary systems.

The research was conducted by an international team of scientists using the European Southern Observatory’s Chile-based Very Large Telescope (VLT).

Led by Armagh Observatory and Planetarium Astronomer, Stefano Bagnulo, the team also comprises John Landstreet, who is Visiting Astronomer of the Armagh Observatory, as well as Professor Emeritus of the Department of Physics and Astronomy at The University of Western Ontario (Canada).

The team also includes Jay Farihi from University College London and Colin Folsom, who started his career in Northern Ireland as a PhD student of the Armagh Observatory and Queen’s University Belfast.

Titled “Discovery of Magnetically Guided Metal Accretion onto a Polluted White Dwarf”*, and published in prestigious The Astrophysical Journal Letters, the research paper outlines how a scar has been found imprinted on the surface of a magnetic white dwarf called WD0816-310.

The object is an Earth-sized remnant of a star in the neighbourhood of our solar system, which has ceased generating energy in its core.

The observed mark is a visible byproduct of the process by which white dwarfs ingest the surrounding planets and asteroids that are born with them.

The scar is composed of a concentration of metals that originate from a planetary fragment as large as or possibly larger than Vesta, which is about 500 kilometres across, and is the second-largest asteroid in our Solar System.

The observations also provide clues to how the star got its metal scar. Astronomers noted that the strength of the metal detection changes as the star rotates, suggesting that the metals are concentrated on a specific area on the star’s surface.

They also found that these changes were synchronised with fluctuations of the white dwarf’s magnetic field and discovered that the metal scar is located on one of the star’s magnetic poles.

This shows that the metals were funnelled onto the star by its magnetic field, creating the scar.**

Stefano Bagnulo, Astronomer at Armagh Observatory and Planetarium, says, “It is well known that some white dwarfs, which are slowly cooling embers of stars like our Sun, are cannibalising pieces of their planetary systems.

“Now we have discovered that the white dwarf’s magnetic field plays a key role in this process, resulting in a scar on its surface,”

To reach these conclusions, the team used the Very Large Telescope’s FORS2 instrument.

Bagnulo continues, “ESO has the unique combination of capabilities needed to observe faint objects such as white dwarfs, and sensitively measure stellar magnetic fields,

Harnessing the power of observations like these allows astronomers to reveal the bulk composition of exoplanets, which are planets orbiting other stars outside the Solar System.

This study also shows how planetary systems can remain dynamically active, even after their ‘death’.

John Landstreet, co-author of the research paper, is member of the team that discovered the first magnetic white dwarf, back in 1970, and is still very active in astronomical research.

He adds: “Surprisingly, the material was not evenly mixed over the surface of the star, as predicted by theory.

“Instead, this scar is a concentrated patch of planetary material, held in place by the same magnetic field that has guided the infalling fragments. Nothing like this has been seen before.”

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