Astronomers using the SMARTS 1.5-meter telescope at the Cerro Tololo Inter-American Observatory in Chile, a program of NSF’s NOIRLab, have discovered the first example of a phenomenally rare type of binary star system, one that has all the right conditions , to eventually trigger a kilonova — the ultra-powerful, gold-producing explosion created by colliding neutron stars. Such an arrangement is so infinitesimal that only about 10 such systems are thought to exist in the entire Milky Way. The results are published in the journal today Nature.
Known as CPD-29 2176, this unusual system is located about 11,400 light-years from Earth. It was first identified by NASA’s Neil Gehrel’s Swift Observatory. Later observations with the 1.5-metre SMARTS telescope allowed astronomers 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 tightly orbiting massive star , which is in the process of forming 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 power of a traditional supernova, which would otherwise “eject” a nearby companion star from the system.
“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,” said Noel D. Richardson of Embry-Riddle Aeronautical University and lead author of the paper. “To someday create a kilonova, the other star would also need to explode as an ultra-stripped supernova, allowing the two neutron stars to eventually collide and merge.”
In addition to discovering an incredibly rare cosmic oddity, finding and studying kilonova progenitor systems like this one can help astronomers unravel the mystery of how kilonovae form and shed light on the origin of the heaviest elements in the universe.
“For quite some time, astronomers have speculated about the exact conditions that could eventually lead to a kilonova,” said NOIRLab astronomer and co-author André-Nicolas Chené. “These new results show that, in at least some cases, two sibling neutron stars can merge if one of them formed without a classic supernova explosion.”
However, the manufacture of such an unusual system is a long and unlikely process. “We know that the Milky Way contains at least 100 billion stars and probably hundreds of billions more. This remarkable binary system is essentially a one-in-ten-billion system,” Chené said. “Prior to our study, it was thought that there should only be one or two such systems in a spiral galaxy like the Milky Way.”
Although this system has the right material to eventually form a kilonova, it will be up to future astronomers to study this event. It will be at least a million years before the massive star ends its life as a titanic supernova explosion, leaving behind a second neutron star. This new stellar remnant and the preexisting neutron star must then gradually coalesce in a cosmic ballet, slowly losing their orbital energy as gravitational radiation.
When they eventually merge, the resulting kilonova blast will create much more powerful gravitational waves, leaving behind a large amount of heavy elements, including silver and gold.
“This system shows that some neutron stars are formed with just a small supernova kick,” Richardson concluded. “By understanding the growing population of systems like CPD-29 2176, we will gain insight into how quiescent some stellar deaths can be and whether these stars can die without traditional supernovae.”