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Anomalous star systems created more fizz and less explosions when they exploded in supernovae. In a lackluster explosion known as a “super-stripping” supernova, researchers discovered two stars 11,000 light-years from Earth.
This is the first confirmed detection of a star system where neutron stars collide and explode, creating kilonovas that release gold and other heavy elements into space. This rare pair of stars is thought to be one of only about 10 such in the Milky Way galaxy. It took a long time to discover.
In 2016, NASA’s Neil Gehrels Swift Observatory detected a large flash of X-ray light emanating from the same region of the sky where a hot, bright Be-type star is located.
Astronomers were curious if the two could be related, so the data were obtained using a 1.5-meter telescope at the Cerro Tololo InterAmerican Observatory in northern Chile. One person who was interested in using this data to learn more about the stars was Dr. Noel D. Richardson, now Assistant Professor of Physics and Astronomy at Embry-Riddle Aeronautical University.
In 2019, college undergraduate Clarissa Pavao approached Richardson while taking an astronomy class and asked if there were any projects he could work on to gain experience in astronomy research. shared telescope data with her, and throughout the pandemic, Pavao learned how to work with data from the Chilean telescope, cleaning it up and reducing distortion.
“Telescopes look at a star and capture all the light so that you can see the elements that make up this star, but Be stars tend to have a disk of material around them,” Pavao said. “It’s hard to see all these things in person.”
She sent Richardson initial results that resembled something like a scatterplot. Tracking observations helped verify the orbit of the binary star system, designated CPD-29 2176.
However, the trajectory was not what they expected. Binary stars usually orbit each other in elliptical orbits. In CPD-29 2176, one star orbits another in a circular pattern that repeats about every 60 days.
Two stars, one big and one small, were orbiting each other in very close orbits. Over time, the larger stars began to release hydrogen and material to the smaller stars, growing from 8-9 times the mass of the Sun to 18-19 times the mass of the Sun.
The primary star grew smaller and smaller while building secondary stars, and by the time it ran out of fuel, it wasn’t enough to create a massive, energetic supernova to eject the remaining material into space. Instead, the explosion was like igniting fireworks.
“The star was so depleted that the explosion didn’t even have enough energy to kick its orbit into the more typical elliptical shape seen in similar binaries,” Richardson said.
What remained of the extremely stripped supernova was a dense remnant known as a neutron star, now orbiting a rapidly rotating massive star. The star pair remains in a stable alignment for about 5 to 7 million years. Both mass and angular momentum have been transferred to the star, ejecting a disk of gas to keep it in balance and keep it from falling apart.
Eventually, the secondary star burns fuel and expands, expelling matter, just like the first star. But that matter cannot easily pile up on a neutron star, so star systems expel matter through space instead. A secondary star can undergo a similar dull supernova and turn into a neutron star.
Over time, perhaps billions of years, the two neutron stars will merge and eventually explode, Kironova releasing heavy elements like gold into space.
“These heavy elements allow us to live as ourselves. For example, most gold was created by supernova relics or neutron star-like stars in the binary star systems we studied. Astronomy deepens our understanding of the world and our place in it,” said Richardson.
“When we look at these objects, we are looking back in time,” Pavao said. “We’re getting to know more about the origins of the universe, and that’s where our solar system is headed. As humans, we started with the same elements as these stars.” A study detailing their findings was published Wednesday in the journal Nature.
Richardson and Pavao also collaborated with physicist Jan J. Eldridge of the University of Auckland, New Zealand, an expert on binary star systems and their evolution. Eldridge considered thousands of models of binary stars, and he estimated that there are likely only 10 across the Milky Way galaxy, similar to those in their study.
The researchers next want to learn more about the Be star itself, and hope to use the Hubble Space Telescope to make follow-up observations. Pavao also plans to graduate and continue to work in astrophysics using the new skills he has acquired. “I never thought I would be working on the evolutionary history of binary star systems and supernovae,” Pavao said. “This was a great project.”
