Queen’s PhD student finds a white dwarf star ripped apart by its companion

A second year PhD student from Queen’s University Belfast has recently published a paper  on the study of an extremely fast evolving astronomical transient, which turns out to be the first of its kind.  

Lanyon 20.07.2018 close-up from side

The paper was written by James Gillanders, a second year PhD student in the Astrophysics Research Centre at the School of Maths  and Physics, and his two PhD supervisors,  Professor Stephen Smartt and Dr. Stuart Sim. 

An explosion in space was discovered in 2018, and a discovery letter was written by another Queen’s student, Owen McBrien. Its origin has been unexplained since then. The trio took another very close look at the spectra of the transient, obtained by the European Southern Observatory, as it evolved. They modelled the spectra with a computer code developed at Queen’s, and from this analysis constrained the abundance of various elements that were produced in the explosion. From this, and other factors, they were able to eliminate various systems that may have produced this explosion, eventually settling on the only plausible scenario, which is the merger of a white dwarf  (a small mass star) and compact object. The compact object must have been either a neutron star or a black hole.  

The  transient was first detected on the night of 28thDecember 2018. It is the first observed merger of its kind, and only the second ever compact merger which has been observed via the electromagnetic spectrum. The white dwarf star was ripped apart by the neutron star/black hole companion and the spectra revealed the chemical elements of oxygen, magnesium, silicon and iron.  

Talking about the discovery, James Gillanders said: “The explosion is likely to be the result of a merger between a white dwarf (the core of a star after it hasevolvedand shed all its outerenvelope) and either the most dense, or second most denseobjectin theknownUniverse (black hole or neutron star,respectively). In this case, two stars were orbiting each other and both evolved in different ways. One collapsed into a black hole or neutron star while the other became a white dwarf. This left a ball about the size of the earth, which was ripped apart as it orbited too close to its companion. We measured the amounts of the different chemical elements ejected as the two objects merged, showing it to be unusually rich in a stable isotope of iron, and the merger model is the only viable explanation.  

Professor Stephen Smartt said: "A white dwarf and a neutron star are two very small, high density stars. Thewhite dwarf is the size of the earth and a neutron star (or black hole) is about10 km in size. As they orbit close to each other they emit gravitationalwaves and lose energy. So, the orbit shrinks over millions of years and finally the white dwarf gets ripped apart as they merge. Only very high-density objects which are in orbit around each other emit such gravitationalwaves. " 

 The paper can be viewed here: https://doi.org/10.1093/mnras/staa1822.

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