First Kilonova Progenitor System identified
Astronomers using the SMARTS 1.5-meter Telescope uncover a one-in-ten-billion binary star system
Peer-Reviewed PublicationIMAGE: THIS IS AN ARTIST’S IMPRESSION OF THE FIRST CONFIRMED DETECTION OF A STAR SYSTEM THAT WILL ONE DAY FORM A KILONOVA — THE ULTRA-POWERFUL, GOLD-PRODUCING EXPLOSION CREATED BY MERGING NEUTRON STARS. THESE SYSTEMS ARE SO PHENOMENALLY RARE THAT ONLY ABOUT 10 SUCH SYSTEMS ARE THOUGHT TO EXIST IN THE ENTIRE MILKY WAY. view more
CREDIT: CTIO/NOIRLAB/NSF/AURA/J. DA SILVA/SPACEENGINE/M. ZAMANI
Astronomers using the SMARTS 1.5-meter Telescope at Cerro Tololo Inter-American Observatory in Chile, a Program of NSF's NOIRLab, have uncovered 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 vanishingly rare that only about 10 such systems are thought to exist in the entire Milky Way Galaxy. The findings are published today in the journal Nature.
This unusual system, known as CPD-29 2176, is located about 11,400 light-years from Earth. It was first identified by NASA’s Neil Gehrels Swift Observatory. Later 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 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.
“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 at Embry-Riddle Aeronautical University and lead author of the paper. “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, shedding light on the origin of the heaviest elements in the Universe.
“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.”
Producing such an unusual system, however, is a long and unlikely process. “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.”
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.
“This system reveals that some neutron stars are formed with only a small supernova kick,” concluded 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.”
More information
This research was presented in the paper “A high-mass X-ray binary descended from an ultra-stripped supernova” to appear in the journal Nature.
The team is composed of Noel D. Richardson (Embry-Riddle Aeronautical University), Clarissa Pavao (Embry-Riddle Aeronautical University), Jan J. Eldridge (University of Auckland), Herbert Pablo (American Association of Variable Star Observers), André-Nicolas Chené (NSF’s NOIRLab/Gemini Observatory), Peter Wysocki (Georgia State University), Douglas R. Gies (Georgia State University), Georges Younes (The George Washington University), and Jeremy Hare (NASA Goddard Space Flight Center).
NSF’s NOIRLab, the US center for ground-based optical-infrared astronomy, operates the International Gemini Observatory (a facility of NSF, NRC–Canada, ANID–Chile, MCTIC–Brazil, MINCyT–Argentina, and KASI–Republic of Korea), Kitt Peak National Observatory (KPNO), Cerro Tololo Inter-American Observatory (CTIO), the Community Science and Data Center (CSDC), and Vera C. Rubin Observatory (operated in cooperation with the Department of Energy’s SLAC National Accelerator Laboratory). It is managed by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with NSF and is headquartered in Tucson, Arizona. The astronomical community is honored to have the opportunity to conduct astronomical research on Iolkam Du’ag (Kitt Peak) in Arizona, on Maunakea in Hawai‘i, and on Cerro Tololo and Cerro Pachón in Chile. We recognize and acknowledge the very significant cultural role and reverence that these sites have to the Tohono O'odham Nation, to the Native Hawaiian community, and to the local communities in Chile, respectively.
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 7, 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.
CREDIT
CTIO/NOIRLab/NSF/AURA/P. Marenfeld
JOURNAL
Nature
METHOD OF RESEARCH
Data/statistical analysis
SUBJECT OF RESEARCH
Not applicable
ARTICLE TITLE
A high-mass X-ray binary descended from an ultra-stripped supernova
ARTICLE PUBLICATION DATE
1-Feb-2023
When your supernova’s a dud: Rare binary star features weirdly round orbit, Embry-Riddle researchers report
IMAGE: BY STUDYING THE BINARY STAR SYSTEM CPD-29 2176 (SHOWN HERE), RESEARCHERS ARE UNRAVELING NEW CLUES TO OUR EARLIEST BEGINNINGS, AS STARDUST. SCIENTISTS ESTIMATE THAT THERE ARE PROBABLY ONLY ABOUT 10 SUCH STAR SYSTEMS IN THE GALAXY AT PRESENT. view more
CREDIT: NOIRLAB/NSF/AURA/J. DA SILVA/SPACEENGINE
After crunching a mountain of astronomy data, Clarissa Pavao, an undergraduate at Embry-Riddle Aeronautical University’s Prescott, Arizona campus, submitted her preliminary analysis. Her mentor’s response was swift and in all-caps. “THERE’S AN ORBIT!” he wrote.
That was when Pavao, a senior Space Physics major, realized she was about to become a part of something big – a paper in the peer-reviewed journal Nature that describes a rare binary star system with uncommon features.
The Nature paper, published on Feb. 1, 2023, and co-authored with Dr. Noel D. Richardson, assistant professor of Physics and Astronomy at Embry-Riddle, describes a twin-star system that is luminous with X-rays and high in mass. Featuring a weirdly circular orbit – an oddity among binaries – the twin system seems to have formed when an exploding star or supernova fizzled out without the usual bang, similar to a dud firecracker.
The binary’s round orbit was a key clue that helped researchers identify the second star in the binary system as a depleted or “ultra-stripped” supernova. Usually, after a star consumes all of its nuclear fuel, its core collapses before exploding into space as a supernova. In this case, Richardson said, “The star was so depleted that the explosion didn’t even have enough energy to kick the orbit into the more typical elliptical shape seen in similar binaries.”
We are Stardust
The binary system’s name sounds like a license plate: CPD-29 2176. Researchers estimate that there are probably only about 10 such star systems in the Galaxy at present. By studying it, they are unraveling new clues to our earliest beginnings, as stardust.
“When we look at these objects, we’re looking backward through time,” explained Pavao. “We get to know more about the origins of the universe, which will tell us where our solar system is headed. As humans, we started out with the same elements as these stars.”
Richardson added that, without binary systems like CPD-29 2176, life on Earth would be very different. “Systems like this are likely to evolve into binary neutron stars, which eventually merge and form heavy elements that get hurled into the universe,” he noted. “Those heavy elements allow us to live the way that we do. For example, most gold was created by stars similar to the supernova relic or neutron star in the binary system that we studied. Astronomy deepens our understanding of the world and our place in it.”
Persistence Pays
The project started when Pavao stopped by Richardson’s office in hopes of scoring a research experience. “I said, `Please give me any research.’” He happened to have data, captured by the Cerro Tololo Interamerican Observatory’s 1.5 meter telescope in Chile, from a bright star known as a Be-type star. The Be star was located at the same location on the sky as another one that had produced a large flash of X-rays. That flash – possibly something called a “soft gamma repeater” – had gotten astronomers’ attention, prompting Richardson and others to request telescope data.
Pavao plotted the spectra of the Be star, but first, she had to clean up the data so they were less noisy. “The telescope looks at a star and it takes in all the light so that you can see the elements that make up this star,” she noted, “but Be stars tend to have discs of matter around them. It’s hard to see directly through all that stuff.”
Persistence paid off: Pavao managed to learn more about data processing and computer coding so that she could analyze the stellar spectra. She and Richardson found one simple line that came from the star and wasn’t influenced by the disc around it. She thought her graph was a scatterplot. Richardson thought otherwise, prompting his all-caps email. After quickly fitting Pavao’s data into a special computer program, he realized they had found an orbit for the star, but it was different than expected. Further data-crunching revealed that one star was indeed tracing a circle around the other one every 60 days or so.
Pavao recalls Richardson saying, “This is not just a simple binary system.”
Collaboration Counts
Enter Jan J. Eldridge of the University of Auckland, a co-author on the Nature paper and a foremost expert on understanding binary star systems and their evolution. At Richardson’s request, Eldridge reviewed thousands of binary star models and found only two that were analogous to the one that he and Pavao were studying.
Eldridge and colleagues then diagramed the life cycle of the two binary system stars, explaining how the supernova relic had puffed up and dumped mass onto the Be star until it began to build up, too. Ultimately, the supernova became a low-mass helium star that exploded, leaving behind a neutron star, but it had already transferred so much of its mass to the Be star that the explosion was lackluster.
“Basically, we found out how the ultra-stripped supernova interacts with the Be star, and how it goes through these weird life-cycle phases,” Pavao explained. “At some point in the future, that Be star will also be a supernova neutron star as the cycle continues. It will become a binary system with two neutron stars, millions of years from now.”
Looking Ahead
A native of Belleville, Illinois, Pavao grew up in a science-focused family. Her father is a computer scientist and her mother is a geologist and amateur astronomer.
During her undergraduate years at Embry-Riddle, Pavao had a chance to complete an undergraduate research experience at the SETI (Search for Extraterrestrial Intelligence) Institute, where she met scientists including Jill Tarter, who was played by actress Jodi Foster in the movie “Contact.”
“It was a life-changing experience,” Pavao said. “Later on in life, I’ll be able to say I went to this observatory and looked for techno-signatures from outer space.” Pavao also credits Richardson with guiding her research and giving her the confidence to succeed. Initially enrolled in a different major, Pavao had the mistaken belief that she was “terrible at math and science” – until she got involved in Richardson’s astronomy project. “He pushes for his students to be on papers,” she noted. “That made a big difference for me.”
With graduation on the horizon next spring, Pavao is evaluating her graduate school options. She’s thinking about a physics focus. “How cool would it be to study dark matter using supercomputers?” she asks.
In addition to Richardson, Pavao and Eldridge, the Nature paper, “A high-mass X-ray binary descended from an ultra-stripped supernova” (Feb. 1, 2023), was co-authored by Herbert Pablo, American Association of Variable Star Observers; André-Nicolas Chené, Gemini Observatory; Peter Wysocki and Douglas R. Gies, CHARA and Georgia State University; Georges Younes, The George Washington University; and Jeremy Hare, NASA Goddard Space Flight Center.
Pavao’s research was supported by Embry-Riddle’s Undergraduate Research Institute and the Arizona Space Grant program. The project also received support from the university’s Faculty Innovative Research in Science and Technology program. Spectroscopy data were collected through NOIR Lab programs 2018B-0137 and 2020A-0054.
The DOI number for this paper is 10.1038/s41586-022-05618-9. Once the embargo has lifted, it will be published by Nature at the following URL: https://www.nature.com/articles/s41586-022-05618-9
JOURNAL
Nature
METHOD OF RESEARCH
Imaging analysis
SUBJECT OF RESEARCH
Not applicable
ARTICLE TITLE
A high-mass X-ray binary descended from an ultra-stripped supernova”
ARTICLE PUBLICATION DATE
1-Feb-2023
COI STATEMENT
There is no conflict of interest. Clarissa Pavao’s research was supported by Embry-Riddle’s Undergraduate Research Institute and the Arizona Space Grant program. The project also received support from the university’s Faculty Innovative Research in Science and Technology program. Spectroscopy data were collected through NOIR Lab programs 2018B-0137 and 2020A-0054.
