When stars die, they are often not alone, and for the first time, astronomers have found a companion to a supernova that lingers long after its siblings destroyed themselves.
The winds of change
In the last moments of a massive star’s life, its interior consists of onion-like layers of elements. In the middle are iron and nickel, the last elements a star can fuse together before it goes to supernova. Its outermost layers are the lightest elements, hydrogen and helium. When the star finally exploding, all these elements are mixed together. Astronomers can study the resulting wreck to find out what the star was made of.
But some supernova remnants lack hydrogen. This indicates that some process removed the outermost layer before the major event. Some older models had predicted that superheated winds from the star blew away the hydrogen layer, but these models could not explain the observations. Another model suggested that these “stripped” supernovae occur when a star has a companion. That companion must be large enough and close enough to use its gravity to suck away hydrogen before the supernova occurs.
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While astronomers had long favored this model, until now they had no observations of companions around a supernova event.
“This was the moment we had been waiting for, to finally see the evidence of a binary system father for a completely stripped-down supernova,” said astronomer Ori Fox of the Space Telescope Science Institute in Baltimore, Maryland. Hubble research program which led to the result. “The goal is to move this area of study from theory to working with data and see what these systems really look like.”
Diamond and supernova
The astronomers found the companion after examining the supernova (SN) 2013ge again. Astronomers had previously studied fading light of the supernova between 2016 and 2020, but more recently the team re-examined it with the Hubble Space Telescope. They found a bright, constant light source where the supernova should have been dimmed a long time ago. It was a companion who shone brightly long after its sibling star had died.
“In recent years, many different pieces of evidence have shown us that stripped-down supernovae are likely to form in binaries, but we had not yet seen the companion. So much of studying cosmic explosions is like forensic science – searching for clues and seeing which theories match. Thanks to Hubble, we can see this directly, says Maria Drout from the University of Toronto, a member of Hubble’s research team.
The observation shows that the scenario for a companion stripping off hydrogen before a supernova is reasonable. But beyond that, this observation provides clues to another type of event: a merging of two exotic objects. The original star left behind either a black hole or a neutron star, and the accompanying star is also large enough to become a supernova and leave a remnant. One day, the two dense objects arrive collides and mergesemits a rage of gravitational waves.
“With the surviving companion to SN 2013ge, we can potentially see the prequel to a gravitational wave event, even if such an event would still be about a billion years in the future,” Fox said.
Hopefully, future observations will reveal more surviving companions, helping astronomers get a complete picture of these processes.
“There is great potential beyond just understanding the supernova itself. Since we now know that most massive stars in the universe are formed in binary pairs, observations of surviving companions are necessary to help understand the details of binary formation, material exchange and co-evolution. “It’s an exciting time to study the stars,” says Fox.
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