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This Binary System is Destined to Become a Kilonova

Kilonovae are extraordinarily rare. Astronomers think there are only about 10 of them in the Milky Way. But they’re extraordinarily powerful and produce heavy elements like uranium, thorium, and gold.

Usually, astronomers spot them after they’ve merged and emitted powerful gamma-ray bursts (GRBs.) But astronomers using the SMARTS telescope say they’ve spotted 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’re the smallest and densest astronomical objects we know of.

Astronomers spotted the progenitor kilonova stars about 11,400 light-years away. They’re named CPD-29 2176 and were first spotted with NASA’s Swift observatory. More observations with the SMARTS 1.5-meter Telescope at the Cerro Tololo Inter-American Observatory in Chile revealed more data.

The findings are in a paper titled “A high-mass X-ray binary descended from an ultra-stripped supernova.” It’s published in the journal Nature. The lead author is Noel D. Richardson, an assistant professor in the Physics and Astronomy Department at Embry-Riddle Aeronautical University.

CPD-29 2176 isn’t a pair of neutron stars, not yet. One of them is a neutron star, and the other is a massive star on its way to exploding as a supernova and leaving a neutron star behind. The stage is set for a kilonova about one million years from now, probably later.

But for the pair of neutron stars to merge as a kilonova in the future, the second star has to explode as a particular type of supernova called an ultra-stripped supernova. One of the reasons that kilonovae are so rare is that ultra-stripped supernovae are so rare. And if that’s not rare enough, the existing neutron star also had to explode as an ultra-stripped supernovae.

When a typical supernova (SN) explodes, it releases a tremendous amount of energy. The explosion can kick its neutron star companion out of the system, eliminating the pathway to a potential kilonova. Eventually, the SN will leave a neutron star behind, but it’ll be alone, and there’ll be no opportunity for two neutron stars to merge and explode as a kilonovae.

But an ultra-stripped supernova (USSN) is different. Ultra-stripped means the SN has experienced extreme mass loss prior to exploding. The mass is lost to its stellar companion, and without that mass, the SN explosion isn’t powerful enough to kick out 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 prior to one exploding as a SN are critical to any eventual kilonova. Changes in mass, stellar rotation, and nuclear burning all determine the eventual core mass of the SN. Under the right but rare conditions, it creates an ultra-stripped supernova.

This is what’s happening in CPD-29 2176, and the researchers doubt the SN will have enough energy when it explodes to eject its neutron star companion. Not only does the current massive star need to explode as a USSN, but the existing neutron star did, too, or else when it exploded as an SN, it would’ve kicked out its stellar companion. So two USSNs are necessary.

“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 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.” This explains why kilonovae are so rare. Mass stripping and weakened SN explosions are prerequisites.

The researchers explained how the system developed so far and what will likely happen in the future.

First, two massive blue stars form in a binary pair. Stars are never the same size; one is always more massive. As the more massive one approaches the end of its life and swells up, the smaller companion is able to siphon off some of the larger star’s material and strip off a significant amount of its outer atmosphere. Then the larger star explodes as an ultra-stripped supernova, but without enough explosive power to kick out its companion, it leaves behind a neutron star.

The next stage is where CPD-29 2176 is now. There’s the neutron star and the larger star that hasn’t exploded yet. The neutron star is siphoning off the star’s outer layers, causing significant mass loss. The tables are turned.

This infographic illustrates the evolution of the star system CPD-29 2176, the first confirmed kilonova progenitor. Stage 1, two massive blue stars form in a binary star system. Stage 2, the larger of the two stars nears the end of its life. Stage 3, the smaller of the two stars siphons off material from its larger, more mature companion, stripping it of much of its outer atmosphere. Stage 4, the larger star forms an ultra-stripped supernova, the end-of-life explosion of a star with less of a “kick” than a more normal supernova. Stage 5, as currently observed by astronomers, the resulting neutron star from the earlier supernova begins to siphon off material from its companion, turning the tables on the binary pair. Stage 6, with the loss of much of its outer atmosphere, the companion star also undergoes an ultra-stripped supernova. This stage will happen in about one million years. Stage 7, a pair of neutron stars in close mutual orbit now remain where once there were two massive stars. Stage 8, the two neutron stars spiral into toward each other, giving up their orbital energy as faint gravitational radiation. Stage 9, the final stage of this system as both neutron stars collide, producing a powerful kilonova, the cosmic factory of heavy elements in our Universe. Image Credit: CTIO/NOIRLab/NSF/AURA/P. Marenfeld

Sometime about a million years in the future, the remaining star will have lost much of its mass and will explode as an ultra-stripped supernovae. It won’t be powerful enough to kick out its neutron star companion. It’ll leave a neutron star behind, and the pair of neutron stars will orbit each other until they spiral inward and eventually merge.

“For quite some time, astronomers speculated about the exact conditions that could eventually lead to a kilonova,” said NOIRLab astronomer and co-author André-Nicolas Chené. “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.”

The odds against this happening are almost overwhelming. But since kilonovae do exist, circumstances must line up to produce them. So every time we witness a kilonova, we’re witnessing a one-in-ten-billion event.

“We know that the Milky Way contains at least 100 billion stars and likely hundreds of billions more. This remarkable binary system is essentially a one-in-ten-billion system,” said Chené. “Prior to our study, the estimate was that only one or two such systems should exist 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 Universe’s heavy elements. So studying them not only reveals details about the events leading up to them but it also helps untangle 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 the orbital radius in orange. The primary star’s supernova event is shown as a vertical dashed line. Before exploding as an ultra-stripped supernova, the primary star’s radius grew, then shrank as the secondary star siphoned off some of its mass. Eventually, the same thing will happen to the secondary star. Image Credit: Richardson et al. 2023.

But humanity will have to survive an awfully long time to see this kilonova event. It could take over a million years for the star to explode as an ultra-stripped supernova. And when it does, the two neutron stars will have to be close enough together before a kilonova can occur. That’s a lot of time and a lot of circumstances.

Now that astronomers have spotted one of these potential kilonova progenitors, they might be in a better position to find more. Along the way, they’ll learn more about ultra-stripped supernovae.

“This system reveals that some neutron stars are formed with only a small supernova kick,” said Richardson. “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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