One in 10 billion: Extremely rare star system is “close” to kilonova

There are only about two handfuls of systems in the Milky Way that end up in a so-called kilonova comparatively soon. Now the first has been discovered.

Artist’s rendering of the two stars (Image: CTIO/NOIRLab/NSF/AURA/J.da Silva/Spaceengine/M.Zamani)

A research team from the United States has discovered an extremely rare binary star system – they say there should only be ten of them in the entire Milky Way. CPD-29 2176 is a small neutron star and a massive blue star in close orbit. In about a million years, the large star will explode as a supernova, leaving two neutron stars – apart from black holes, the densest objects in the cosmos. The team explains that they will then collide and end up in a so-called kilonova. Such explosions produce particularly heavy elements such as gold and silver.

The composition of the system of neutron stars and giant stars is so “vanishingly rare” that the research team assumed before the discovery that there would only be one or two of the at least 100 billion stars in the Milky Way. This assumption has now been corrected upwards, but it is still an exceptional find. The team explains that such a system forms when two massive blue giant stars form a pair.

As the larger of the two stars nears its end and the smaller then pulls material towards it, the remaining core explodes in a particularly faint “ultra-stripped supernova”. What remains is a system like CPD-29 2176. Next, in a million years at the earliest, the neutron star will again divert material from the giant star until it also ends in such an explosion. Two neutron stars remain, which eventually collide and die in a kilonova.

The development of such a system, we see it in stage 5. (Image: CTIO/NOIRLab/NSF/AURA/P. Marenfeld)

Kilonovae are about 1000 times brighter than normal novae, but significantly fainter than supernovae. The process was first described theoretically a few decades ago, and the first direct detection was achieved in 2017 via the gravitational waves produced in the process. Later it was shown that it actually synthesized elements heavier than iron. Thus, the last piece of the puzzle had been inserted into the picture of the formation of the elements. At CPD-29 2176, the immediate prehistory can now be explored, so to speak. The system is 11,400 light years distant.

The discovered system is so extraordinary, for example, because the two stars orbit so closely despite the explosion of one, explains research leader Noel Richardson. Actually, it would be expected that the explosion of one star would throw the other out of the system. The comparatively weak “ultra-stripped supernova” would not be able to do this. Using CPD-29 2176, it is now possible to investigate how quiet some stellar explosions can be, the research team adds. Her work has been published in the scientific journal Nature.

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