Kilonovae are extremely rare. Astronomers believe there are only about 10 of them in the Milky Way. But they are extraordinarily powerful, producing heavy elements like uranium, thorium, and gold.
Usually, astronomers discover them after they have merged and emitted strong gamma-ray bursts (GRBs). However, astronomers using the SMARTS telescope say they have discovered a kilonova progenitor for the first time.
A kilonova explosion occurs when two neutron stars—or a neutron star and a black hole—merge. Neutron stars are the stellar remnants of massive stars that explode as supernovae. They are the smallest and densest astronomical objects that we know.
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Astronomers spotted the kilonova progenitor stars about 11,400 light-years away. Called CPD-29 2176, they were first spotted by NASA’s Swift Observatory. Further observations with the SMARTS 1.5-meter telescope at the Cerro Tololo Inter-American Observatory in Chile yielded more data.
The results are published in a paper entitled “A high-mass X-ray binary derived from an ultra-stripped supernova”. It was published in the journal Nature. The lead author is Noel D. Richardson, Assistant Professor in the Department of Physics and Astronomy at Embry-Riddle Aeronautical University.
CPD-29 2176 is not a pair of neutron stars, not yet. One of them is a neutron star, the other a massive star that is on the way to exploding as a supernova and leaving behind a neutron star. The conditions for a kilonova in about a million years, probably later, have been created.
But in order for the pair of neutron stars to merge as a kilonova in the future, the second star must explode as a specific type of supernova called an ultra-stripping supernova. One of the reasons kilonovae are so rare is that ultra-striped supernovae are so rare. And as if that wasn’t rare enough, the existing neutron star also had to explode as ultra-stripped supernovae.
When a typical supernova (SN) explodes, it releases an enormous amount of energy. The explosion can knock its neutron star companion out of system, eliminating the path to a potential kilonova. Eventually the SN will leave behind a neutron star, but it will be alone and there will be no opportunity for two neutron stars to merge and explode as kilonovae.
But an Ultra-Stripped Supernova (USSN) is different. Ultra-stripped means the SN experienced an extreme mass loss prior to the explosion. The mass is lost to its stellar companion, and without that mass the SN explosion isn’t powerful enough to eject its companion when the SN explodes. These are important details because most stars massive enough to explode as SN exist in binary pairs.
The interactions between the pair of stars before one explodes as an SN are critical to any potential kilonova. Changes in mass, stellar rotation, and nuclear burn all determine the eventual core mass of the SN. Under the right but rare conditions, it produces an ultra-stripped supernova.
This is happening in CPD-29 2176, and researchers doubt the SN will have enough energy when it explodes to eject its neutron star companion. Not only the current massive star has to explode as USSN, but also the existing neutron star, otherwise it would have thrown out its stellar companion in the explosion as SN. Two USSNs are therefore required.
“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 lead author Richardson. “To someday 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.” This explains why kilonovas are so rare. Mass stripping and weakened SN explosions are prerequisites.
The researchers explained how the system has evolved so far and what is likely to happen in the future.
First, two massive blue stars form in a binary pair. Stars are never the same size; one is always massive. As the more massive star approaches the end of its life and swells, the smaller companion is able to siphon off some of the larger star’s material and shed a significant portion of its outer atmosphere. Then the larger star explodes as an ultrathin supernova, but without enough explosive power to knock out its companion, leaving a neutron star in its wake.
The next level is where CPD-29 2176 is now. There is the neutron star and the larger star that has not yet exploded. The neutron star siphons off the star’s outer layers, resulting in significant mass loss. The tables are turned.
This infographic illustrates the evolution of star system CPD-29 2176, the first confirmed kilonova progenitor. Stage 1, two massive blue stars are forming in a binary star system. Stage 2, the larger of the two stars is nearing the end of its life. Level 3, the smaller of the two stars, siphons material from its larger, more mature companion, stripping much of its outer atmosphere. Stage 4, the larger star forms an ultra-stripped supernova, the explosion of a star at the end of its life with less “kick” than a more normal supernova. Stage 5, as currently observed by astronomers, the resulting neutron star from the earlier supernova begins siphoning material from its companion, turning the tables on the binary pair. Stage 6, with the loss of much of its outer atmosphere, the companion also undergoes an ultra-stripped supernova. This stage will occur in about a million years. Stage 7, a pair of neutron stars in close mutual orbit, now remains where two massive stars once were. Stage 8, the two neutron stars spiral towards each other, releasing their orbital energy as weak gravitational radiation. Stage 9, the last stage of this system, when both neutron stars collide and create a powerful kilonova, the cosmic factory of heavy elements in our universe. Credit: CTIO/NOIRLab/NSF/AURA/P. Marenfeld
Sometime about a million years into the future, the surviving star will have lost much of its mass and will explode as ultra-stripped supernovae. It won’t be powerful enough to knock out its neutron star companion. It will leave behind a neutron star, and the two neutron stars will orbit each other until they spiral inward and eventually merge.
“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.”
The odds of this happening are almost overwhelming. But since there are kilonovae, the circumstances have to be right to produce them. So every time we witness a kilonova, we are witnessing an event out of one in ten billion.
“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.”
There’s more to Kilonovae than gravitational waves and a massive explosion. These events are also a source of the heavy elements of the universe. So their investigation not only reveals details about the events that led up to them, but also helps unravel the history of nucleosynthesis.
This figure from the study shows the stellar radii (blue for the secondary star and red for the primary star) and orbital radius in orange. The primary star’s supernova event is shown as a vertical dashed line. Before exploding as an ultrathin supernova, the primary star’s radius grew and then contracted as the secondary star shed some of its mass. Eventually the same will happen to the secondary star. Photo credit: Richardson et al. 2023
But humanity will have to survive an awfully long time to see this kilonova event. It could be over a million years before the star explodes as an ultra-stripped supernova. And if this is the case, the two neutron stars must be close enough before a kilonova can form. That’s a lot of time and a lot of trouble.
Now that astronomers have spotted one of these potential kilonova progenitors, they might be in a better position to find others. Along the way, they learn more about ultra-stripping supernovae.
“This system shows that some neutron stars are formed with just a small supernova kick,” Richardson said. “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.”