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An unusual star system created more fizz and less explosion when it exploded in a supernova.
The lackluster explosion, known as an “ultra-stripped” supernova, led researchers to discover the two stars 11,000 light-years away from Earth.
It is 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 10 like it in the Milky Way galaxy.
The discovery was a long time coming.
In 2016, NASA’s Neil Gehrels Swift Observatory detected a large flash of X-ray light, originating in the same region of the sky as a hot, bright Be-type star.
The astronomers were curious if the two could potentially be linked, so the data was captured using the 1.5-meter telescope at the Cerro Tololo Inter-American Observatory in northern Chile.
One of those 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 taking her astronomy class to ask if she had any projects she could work on to gain experience in astronomical research. She shared the telescope data with her, and during the pandemic, Pavao learned how to work with the data from the telescope in Chile and clean it up to reduce distortion.
“The telescope looks at a star and captures all the light so you can see the elements that make up this star, but Be stars tend to have disks of matter around them,” Pavao said. “It’s hard to see straight through all that stuff.”
He sent his initial results, which looked something like a scatter plot, to Richardson, who acknowledged that he had fixed 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 that orbit was not what they expected. Binary stars usually revolve around each other in an oval-shaped orbit. In CPD-29 2176, one star orbits the other in a circular pattern that repeats 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 had begun to shed its hydrogen, releasing material onto the smaller star, which grew from 8 to 9 times the mass of our sun to 18 to 19 times the mass of our sun, Richardson said. For the sake of comparison, the mass of our sun is 333,000 times that of Earth.
The primary star got smaller and smaller as it built the secondary star, and when it used up all its fuel, there wasn’t enough to create a massive, energetic supernova to release the remaining material into space.
Instead, the explosion was like lighting a failed firework.
“The star was so depleted that the explosion didn’t even have enough energy to propel (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 spinning massive star. The stellar pair will remain in a stable configuration for about 5 to 7 million years. Because both mass and angular momentum have been transferred to the star Be, it releases a disk of gas to keep it balanced and make sure it doesn’t break apart.
Eventually, the secondary star will also burn its fuel, expand, and release material as the first did. But that material can’t be easily piled up in the neutron star, so the star system will instead release the material through space. The secondary star will likely experience a similar lackluster supernova and become a neutron star.
Over time, meaning probably a couple of billion years, the two neutron stars will merge and eventually explode into a kilonovareleasing heavy elements like gold into the universe.
“Those heavy elements allow us to live the way we do. For example, most of the gold was created by stars similar to the relic supernova 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 may know more about the origins of the universe, which will tell us where our solar system is headed. As humans, 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 in binary star systems and their evolution. Eldridge reviewed thousands of models of binary stars and estimated that there are likely only 10 in the entire Milky Way galaxy similar to the one he studied.
Next, the researchers want to work to learn more about the star Be and hope to make follow-up observations using the Hubble Space Telescope. Pavao also has his sights set on graduating and continuing to work in space physics research using the new skills he has acquired.
“I never thought that I would be working on the evolutionary history of binary star systems and supernovae,” Pavao said. “It has been an incredible project.”
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