An incredibly rare star system that will eventually tear itself apart in a 'kilonova' – a powerful explosion which creates heavy metals such as gold – has been spotted for the first time.
Gold and other heavy elements like platinum are produced during kilonova explosions, where two neutron stars merge.
Neutron stars are the burnt-out cores of dead stars, and are so dense that a handful of their material would weigh as much as Mount Everest.
These systems are so phenomenally rare that only about 10 are thought to exist in our entire Milky Way galaxy.
Astronomers spotted the unusual system known as CPD-29 2176, which is located about 11,400 light-years from Earth.
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Observations with the SMARTS 1.5-metre Telescope allowed them to deduce the orbital characteristics and types of stars that make up this system – a neutron star created by an ultra-stripped supernova and a closely orbiting massive star that is in the process of becoming an ultra-stripped supernova itself.
An ultra-stripped supernova is the end-of-life explosion of a massive star that has had much of its outer atmosphere stripped away by a companion star.
This class of supernova lacks the explosive force of a traditional supernova, which would otherwise 'kick' a nearby companion star out of the system.
Lead author Noel D Richardson, of Embry-Riddle Aeronautical University in the US, said: "The current neutron star would have to form without ejecting its companion from the system.
"An ultra-stripped supernova is the best explanation for why these companion stars are in such a tight orbit.
"To one day create a kilonova, the other star would also need to explode as an ultra-stripped supernova so the two neutron stars could eventually collide and merge."
As well as representing the discovery of an incredibly rare cosmic oddity, finding and studying kilonova progenitor systems such as this can help astronomers unravel the mystery of how kilonovae form.
This will shed light on the origin of the heaviest elements in the universe, like gold and platinum.
"For quite some time, astronomers speculated about the exact conditions that could eventually lead to a kilonova," said NOIRLab astronomer and co-author Andre-Nicolas Chene.
"These new results demonstrate that, in at least some cases, two sibling neutron stars can merge when one of them was created without a classical supernova explosion."
Though this system has all the right stuff to eventually form a kilonova, it will be up to future astronomers to study that event.
It will take at least one million years for the massive star to end its life as a titanic supernova explosion and leave behind a second neutron star.
This new stellar remnant and the pre-existing neutron star will then need to gradually draw together in a cosmic ballet, slowly losing their orbital energy as gravitational radiation.
When they eventually merge, the resulting kilonova explosion will produce much more powerful gravitational waves and leave behind in its wake a large amount of heavy elements, including silver and gold.
Richardson said: "This system reveals that some neutron stars are formed with only a small supernova kick.
"As we understand the growing population of systems like CPD-29 2176 we will gain insight into how calm some stellar deaths may be and if these stars can die without traditional supernovae."
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