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An unusual star system created more eruptions and fewer explosions when it exploded in a supernova.
Known as an “ultra-striped” supernova, the lifeless explosion prompted researchers to discover two stars 11,000 light-years from Earth.
This is the first confirmed detection of a star system that will one day create a kilonova in which neutron stars collide and explode, releasing gold and other heavy elements into space. The rare pair of stars is believed to be one of about 10 such in the Milky Way galaxy.
The discovery was long ago.
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.
Astronomers wondered if the two could potentially be linked, so the data was collected using the 1.5-metre telescope of the Cerro Tololo Inter-American Observatory in northern Chile.
One of those interested in using this data to learn more about the star is Dr. Noel D. Richardson is currently an assistant professor of physics and astronomy at Embry-Riddle Aeronautical University.
In 2019, Clarissa Pavao, an undergraduate at the university, approached Richardson in her astronomy class, asking if she had any projects she could work on to gain experience in astronomy research. She shared the telescope data with him, and during the pandemic, Pavao learned how to work with data from the telescope in Chile and how to clean it up to reduce distortion.
“The telescope looks at a star and takes in 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. Said. “It’s hard to see directly through all this stuff.”
He sent his first results, similar to a scatterplot, to Richardson, who noticed that he had detected an orbit for the binary star system. Follow-up observations helped them confirm the orbit of the binary star system CPD-29 2176.
But this trajectory was not what they expected. Typically, binary stars revolve around each other in an oval-shaped orbit. On CPD-29 2176, one star orbits the other in a circular pattern that repeats about every 60 days.
Two stars, one larger and the other smaller, were orbiting each other in very close orbits. Over time, the larger star began to shed its hydrogen, and the smaller star’s mass increased from 8 or 9 times that of our sun to 18 or 19 times that of our sun, Richardson said. For comparison, the mass of our sun is 333,000 times that of the Earth.
The main star shrank and shrank as it formed the secondary star – and when it had used up all of its fuel, there was not enough of it to release its remaining material into space, creating a massive, energetic supernova.
Instead, the explosion was like lightning and fireworks.
“The star was so extinct that it didn’t even have enough energy to make the blast’s trajectory into the more typical elliptical shape seen in similar binaries,” Richardson said.
What remained after the ultra-stripped supernova was a dense remnant known as a neutron star, now orbiting the rapidly spinning massive star. The star pair will remain in a stable configuration for about 5 to 7 million years. As both mass and angular momentum are transferred to the Be star, it releases a disk of gas to keep it stable and ensure it doesn’t tear itself apart.
Eventually, the secondary star will also burn its fuel, expand and release material just as the first one did. However, this material cannot easily accumulate on the neutron star, instead the star system will release the material into space. The secondary star will likely experience a similar lifeless supernova and turn into a neutron star.
In time, that is, possibly in a few billion years, the two neutron stars will merge and eventually an explosion will occur. kilonovareleases heavy elements such as gold into the universe.
“These heavy elements allow us to live the way we do. For example, much of the gold was created by stars similar to the supernova remnant or neutron star in the binary system we’re studying. Astronomy deepens our understanding of the earth and our place in it,” said Richardson.
“When we look at these objects, we’re looking back in time,” Pavao said. “We’re learning more about the origins of the universe, which will show us where our solar system is headed. As humans, we set out with the same elements as these stars.”
A study detailing its findings, published Wednesday in the journal Nature.
Richardson and Pavao also worked with physicist Jan J. Eldridge, an expert on binary star systems and their evolution at the University of Auckland in New Zealand. Eldridge reviewed thousands of binary star models and estimated that there are probably only 10 in the entire Milky Way galaxy similar to the ones he studied.
Next, the researchers want to work to learn more about the Be star itself and hope to make follow-up observations using the Hubble Space Telescope. Pavao is also focusing on graduating and continuing to work on space physics research using her newly acquired skills.
“I never thought I’d be studying the evolutionary history of binary star systems and supernovas,” Pavao said. “It’s been a great project.”
