Astronomers have discovered the first example of an extremely unusual type of binary star system that meets all the right conditions for a kilonova to be unleashed sooner or later, that is, an ultra-powerful explosion, producing gold and other heavy chemical elements, which It is formed by the collision and merger between neutron stars. These conditions are so unusual that there are thought to be only about 10 similar systems in the entire Milky Way galaxy. The international team of astronomers that made the discovery used the 1.5-meter SMARTS Telescope at Cerro Tololo, Chile, run by the NOIRLab of the NSF (United States National Science Foundation).
The unusual star system, known as CPD-29 2176, is located about 11,400 light-years from Earth and was first identified by NASA’s Neil Gehrels Swift Observatory. Subsequent observations with the SMARTS 1.5-meter Telescope allowed astronomers to deduce the orbital characteristics and types of stars that make up this system: a neutron star created by an ultranaked supernova and a closely orbiting massive star in the process of becoming a an ultra-naked supernova by itself. An ultranaked supernova is the explosive end 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 expel a nearby companion star from the system.
“The current neutron star formed without ejecting its companion system. An ultranaked supernova is the best explanation for why these companion stars are in such a tight orbit,” said lead author of the research Noel D. Richardson of Embry-Riddle Aeronautical University in the United States. “To create a kilonova one day, the other star would also have to explode as an ultranaked supernova, so the two neutron stars could collide and merge.”
In addition to representing the discovery of an incredibly unusual cosmic oddity, finding and studying kilonova parent systems like this one may help astronomers unravel the mystery of how kilonovae form, giving clues to the origin of the heaviest elements in the universe.
“For quite some time, astronomers have speculated about the exact conditions that could eventually produce a kilonova. These new results demonstrate, at least in some cases, that two sister neutron stars can merge when one of them was created without a classical supernova explosion,” said André-Nicolas Chené, NOIRLab astronomer and co-author of the study. However, forming such an unusual system is a long and unlikely process. “We know that the Milky Way contains at least a hundred billion stars and probably hundreds of billions more. This remarkable binary system is essentially a one in ten billion system,” Chené added. “Before our study, the estimate was that only one or two of these systems should exist in a spiral galaxy like the Milky Way,” he concluded.
Although this system has everything necessary to end up forming a kilonova, it will be up to future astronomers to study this event. It will take at least a million years for the massive star to end its life with a titanic supernova explosion and become a second neutron star. This new stellar remnant and the pre-existing neutron star will eventually join after a gradual process of rapprochement, comparable in some ways to a kind of cosmic ballet, slowly losing their orbital energy in the form of gravitational radiation. When they merge, the resulting kilonova explosion will produce much more powerful gravitational waves and create a large amount of heavy chemical elements, including silver and gold. “This system reveals that some neutron stars form only with a small supernova impulse. As we learn more about star systems like CPD-29 2176, we’ll have a better idea of how quiet some stellar deaths can be and whether these stars can die without generating traditional-type supernova explosions.” Richardson concluded. The study is titled “A high-mass X-ray binary descended from an ultra-stripped supernova”. And it has been published in the academic journal Nature. (Source: NOIRLab. CC BY 4.0)