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An unusual star system made more buzz and less bang when it exploded in a supernova.
The faint explosion, known as an “ultra-abstract” 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 kilonovas – when neutron stars collide and explode, launching gold and other heavy elements into space. The rare stellar pair is thought to be one of only about 10 such in the Milky Way.
Discovery has been coming for a long time.
In 2016, NASA’s Neil Gehrels Swift Observatory detected a large flash of X-ray light, which originated from the same region in the sky as a hot, bright Be star.
Astronomers were curious whether the two could be linked, so the data was captured using the Cerro Tololo Inter-American Observatory’s 1.5-meter telescope in northern Chile.
One interested in using this data to learn more about the star has been Dr. Noel Richardson, who is now an assistant professor of physics and astronomy at Embry-Riddle Aeronautical University.
In 2019, Clarissa Pavao, an undergraduate at the university, called Richardson while attending an astronomy class to ask if he had any projects she could work on to gain experience in astronomy research. He shared telescope data with her and throughout the pandemic, Pavao learned how to work with data from the telescope in Chile and clean it up to reduce distortion.
“A telescope looks at a star and takes in all the light so you can see what elements that star is made of — but Be stars tend to have disks of matter around them,” Pavao said. “It’s hard to see all of these things firsthand.”
She sent her preliminary results—which looked something like a scatterplot—to Richardson, who realized she had determined an orbit for the double star system. Follow-up observations helped them verify the orbit of the binary star system, called CPD-29 2176.
But this orbit was not what they expected. Normally, binary stars orbit each other in an elliptical orbit. In CPD-29 2176, one star orbits the other in a circular pattern that repeats about every 60 days.
The two stars, one larger and the other smaller, were orbiting each other in a very close orbit. Over time, the larger star begins to shed its own hydrogen, releasing material on the smaller star, which grows from 8 or 9 times the mass of our Sun to 18 or 19 times the mass of our Sun, Richardson said. For comparison, the mass of the Sun is 333,000 times that of the Earth.
The main star got smaller and smaller as the secondary star was being built—and by the time it had used up all of its fuel, there wasn’t enough to create a massive, energetic supernova to launch the remaining material into space.
Instead, the explosion was more like a failed firework igniting.
“The star was so depleted that the explosion didn’t have enough energy to drive its orbit into the typical elliptical shape seen in similar binaries,” Richardson said.
What was left after the supernova was a dense remnant known as a neutron star, which now orbits the massive, rapidly spinning star. The star 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 Be star, it releases a disc of gas to maintain balance and ensure it doesn’t tear itself apart.
Eventually, the secondary star will also burn up with its fuel, expanding and releasing material as the first star did. But this matter could not easily accumulate on the neutron star, so instead, the star system would shoot the matter out through space. The secondary star is likely to experience a similarly faint supernova and transform into a neutron star.
Over time—most likely a few billion years—the two neutron stars will merge and eventually explode into a radius. kilonovareleases heavy elements such as 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 resembling the remnants of a supernova or a neutron star in the binary system we studied. Astronomy deepens our understanding of the world and our place in it.”
“When we look at these things, we’re looking back through time,” Pavao said. “We are learning more about the cosmogony, which is going to tell us where our solar system is going. As humans, we started out with the same elements as these stars.”
A study detailing their findings was published Wednesday in the journal nature.
Richardson and Pavau also worked with physicist Jean J. Eldridge at the University of Auckland in New Zealand, who is an expert on binary star systems and their evolution. Eldridge reviewed thousands of binary star models and estimated that there are likely only 10 in the entire Milky Way similar to the ones in their study.
Next, the researchers want to work on learning more about the Be Star itself, and hope to make follow-up observations with the Hubble Space Telescope. Pavau also has her sights set on graduating – and continues to work in astrophysics research using the new skills she has acquired.
“I never thought I would be working on the evolutionary history of binary star systems and supernovae,” said Pavau. “It was an amazing project.”
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