‘Zombie’ stars on the run

FAU-astronomers discovered survivors of supernova explosions

An international consortium led by FAU astronomers has discovered three hyper-velocity stars (HVS), which have survived supernova explosions. These ‘zombie’ stars could give clues about how chemical elements are created and distributed in the Universe. The researchers have not only discovered a new class of HVS, they have also identified a new physical slingshot mechanism for ejecting the stars.

First discovered in 2005, hyper-velocity stars move through space at such a high speed that they will escape from our galaxy at some point. Despite an intense search no more than two dozen HVS have been discovered up to now. How these stars gain their enormous momentum to overcome the gravitational attraction of the galaxy is still under discussion. ‘The most popular explanation is that the monster black hole in the centre of the Milky Way disrupts a binary star that comes too close,’ explains Prof. Dr. Ulrich Heber from the Institute of Astronomy at FAU, one of the researchers to first discover high velocity stars. ‘More recent investigations, however, have shown that there have to be alternative slingshot mechanisms to explain the diversity of HVSs,’ his FAU colleague, Dr. Andreas Irrgang, adds.

A new class of star discovered

The Gaia space observatory of the European Space Agency (ESA) has paved the way to understanding the origin of HVSs. Gaia’s astrometric data, published in April 2018, allowed the three dimensional trajectories of HVSs through the Milky Way to be calculated for the first time and the place of origin to be identified. FAU astronomer, Dr Roberto Raddi, cross-matched the Gaia data with other astronomical catalogues to search for new HVSs and made an astonishing discovery: three new HVSs that are strikingly similar to the exotic HVS, LP 40-365, which was netted serendipitously two years before by another team. An international collaboration consisting of astronomers from ten universities in Germany, the UK, the USA, and Italy carried out an extensive observational campaign with large telescopes, including the Hubble Space Telescope and the European Very Large Telescope. The three candidates are as similar to LP 40-365 as peas in a pod: a new class of high velocity star was found.

The chemical composition is unique

The most important result was the unique composition of the new stars. They consist mostly of neon and oxygen, with no trace of hydrogen and helium, which are the dominant constituents of normal stars like the Sun. How is this possible? ‘Explosive thermonuclear explosions, such as in a hydrogen-bomb, may transform light chemical elements into heavier ones by nuclear fusion,’ Dr Roberto Raddi explains. ‘Indeed, this has been proven to happen in thermonuclear supernovae caused by the explosion of a so-called white dwarf, an Earth-sized degenerate star. A white dwarf may explode if it accretes matter from a companion star,’ he adds.

Zombie stars survived supernova

Did FAU astronomers detect zombie-dwarfs, survivors of a supernova? Previous numerical simulations suggested that such an explosion would destroy the white dwarf completely. The former companion would be left behind and then ejected from the Milky Way at hyper-speed. New models, however, revealed that in specific conditions the white dwarf is not destroyed entirely. About 20% of the mass may remain, consisting of neon, oxygen, magnesium, aluminum, and heavier elements, like manganese, iron, and nickel. This corresponds precisely to the chemical cocktail found in the type LP 40-365 HVSs. However, it remained unclear why these relics of a stellar death contain no carbon, which they should have according to the numerical models. ‘This is one of the open questions which still have to be answered,’ Roberto Raddi says.

Supernova explosion creates an HVS pair

How is the white dwarf relic ejected – and what happens to the companion star? The researchers came up with a plausible explanation. The stellar companion had to be very close to the white dwarf for mass to be transferred to the latter, causing an explosion. This means both stars would be required to orbit their common centre of mass at extreme velocities. When the white dwarf exploded, it received a kick so strong that it broke the link between the binary stars, causing both partners to fly out in different directions at hyper-speed. ‘Actually, two hyper-velocity stars were launched at the same time,’ Ulrich Heber concluded. ‘Unfortunately, it will be very difficult to find the former companion star to any of the zombie dwarfs, because according to our estimates the ejection happened already 40 million years ago.’

The team has successfully discovered a new class of HVSs as well as identifying a new physical slingshot mechanism for HVSs. The results have been accepted for publication in the renowned scientific journal Monthly Notices of the Royal Astronomical Society

The journal Nature has reported the paper as a research highlight.

Further Information

Prof. Dr. Ulrich Heber
Tel.: 0951/95222-14
ulrich.heber@fau.de