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An unusual star system created more excitement and less bang when it exploded into a supernova.
The dull explosion, known as an “ultra-stripped” supernova, led researchers to discover the two stars 11,000 light-years from Earth.
It’s the first confirmed detection of a star system that will one day create a kilonova – when neutron stars collide and explode, releasing gold and other heavy elements into space. The rare stellar pair is believed to be one of only 10 alike in the Milky Way galaxy.
The discovery was slow in coming.
In 2016, NASA’s Neil Gehrels Swift Observatory detected a large flash of X-rays, which came from the same region of the sky where a hot, bright Be-type star was.
Astronomers were curious if the two could potentially be linked, so the data was captured using the 1.5-meter telescope at the Inter-American Cerro Tololo Observatory in northern Chile.
One of the people interested in using this data to learn more about the star was Dr. Noel D. Richardson, now an assistant professor of physics and astronomy at Embry-Riddle Aeronautical University.
In 2019, Clarissa Pavao, an undergraduate student at the university, approached Richardson while he was taking his astronomy class to ask if he had any projects she could work on to gain experience in science. astronomy research. He shared telescope data with her and throughout the pandemic Pavao learned to work with telescope data in Chile and clean it to reduce distortion.
“The telescope looks at a star and it picks up all the light so you can see the elements that make up that star – but Be stars tend to have disks of matter around them,” Pavao said. “It’s hard to see straight through it all.”
She sent her first results – which looked something like a scatter plot – to Richardson, who acknowledged that she had pinned an orbit for the double star system. Follow-up observations helped them verify the orbit of the binary star system, named CPD-29 2176.
But this orbit was not what they expected. Typically, binary stars swirl around each other in an oval-shaped orbit. In CPD-29 2176, one star orbits the other in a circular pattern that repeats approximately every 60 days.
The two stars, one larger and one smaller, revolved around each other in a very close orbit. Over time, the larger star began to release its hydrogen, releasing material onto the smaller star, which went from 8 or 9 times the mass of our sun to 18 or 19 times the mass of our sun, a said Richardson. For comparison, the mass of our Sun is 333,000 times that of Earth.
The primary star grew smaller and smaller as the secondary star built – and by the time it had used up all of its fuel, there was not enough left to create a massive, energetic supernova to release its material remaining in space.
Instead, the explosion was like igniting a failed firework.
“The star was so exhausted that the explosion didn’t even have enough energy to launch (its) orbit into the more typical elliptical shape seen in similar binaries,” Richardson said.
What was left after the ultra-stripped supernova was a dense remnant known as a neutron star, which now orbits the rapidly rotating massive star. The star pair will remain in a stable configuration for about 5-7 million years. Because mass and angular momentum have been transferred to the Be star, it releases a disk of gas to maintain balance and ensure it doesn’t tear apart.
Eventually, the secondary star will also burn its fuel, expand, and release material as the first did. But this material cannot be easily piled onto the neutron star, so instead the star system will release the material through space. The secondary star will likely experience a similar dull supernova and transform into a neutron star.
Over time – that is, probably a few billion years – the two neutron stars will merge and eventually explode in a kilonovareleasing heavy elements like gold into the universe.
“These heavy elements allow us to live as we do. For example, most of the gold was created by stars similar to the supernova relic or neutron star in the binary system we studied. Astronomy deepens our understanding of the world and our place in it,” Richardson said.
“When we look at these objects, we are looking back in time,” Pavao said. “We are learning more about the origins of the universe, which will tell us where our solar system is heading. As human beings, we started with the same elements as these stars.
A study detailing their findings published Wednesday in the journal Nature.
Richardson and Pavao also worked with physicist Jan J. Eldridge of the University of Auckland in New Zealand, an expert on binary star systems and their evolution. Eldridge looked at thousands of binary star patterns and estimated that there were probably only 10 in the entire Milky Way galaxy similar to the one in their study.
Next, the researchers want to learn more about the Be star itself and hope to make follow-up observations using the Hubble Space Telescope. Pavao also plans to graduate and continue working on space physics research using the new skills she learned.
“I never thought I would work on the evolutionary history of binary star systems and supernovae,” Pavao said. “It was an amazing project.”