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Red giant and white dwarf
This artist’s view shows a white dwarf star accumulating material from a nearby red giant star. Ultimately, the white dwarf erupts into a supernova. (Instituto de Astrofísica de Canaria Illustration / Romano Corradi)

In what sounds like a cosmic episode of “CSI,” sleuthing astronomers have figured out what touched off a stellar explosion 545 million light-years away, based on evidence left behind at the scene of the crime.

An international team of astronomers used the Hubble Space Telescope and other observatories to sift through the chemical fingerprints left behind in the remnants of a Type Ia supernova known as SN 2015cp. The astronomers knew the type of star that blew up: It was a carbon-oxygen white dwarf. But they wanted to find out whether a different kind of star had a hand in the blast.

Today the astronomers reported the detection of hydrogen-rich debris in the vicinity of the supernova site — which cracks the case wide open.

“The presence of debris means that the companion was either a red giant star or similar star that, prior to making its companion go supernova, had shed large amounts of material,” University of Washington astronomer Melissa Graham said in a news release.

Graham is the lead author of a paper on the findings accepted for publication in The Astrophysical Journal. She presented the team’s results today at the American Astronomical Society’s winter meeting.in Seattle.

Type Ia supernovae are of special interest because they have a consistent luminosity, which makes them useful to astronomers as “standard candles” to judge cosmic distances. Observations of such supernovae were key to the discovery in the late 1990s that the expansion of the universe is accelerating, due to a mysterious phenomenon known as dark energy.

However, there’s much that still isn’t known about Type Ia supernovae.

Astronomers think most of the stellar blowups are triggered by the interaction between two carbon-oxygen white dwarfs in a binary star system. Such stars are small, dense and tidy. In contrast, red giants are big and messy. If a red giant is paired with a white dwarf in a binary system, the hydrogen-rich material thrown off by the bigger star could fall onto the smaller one, eventually sparking an explosion.

Figuring out the detailed mechanisms that trigger Type Ia supernovae is key to building astronomers’ confidence in their use as standard cosmological candles.

“All of the science to date that has been done using Type Ia supernovae … rests on the assumption that we know reasonably well what these ‘cosmic lighthouses’ are and how they work,” Graham said. “It is very important to understand how these events are triggered, and whether only a subset of Type Ia events should be used for certain cosmology studies.”

To get a better understanding of the trigger mechanisms, Graham and her colleagues surveyed 70 Type Ia supernovae one to three years after their blowups. “We were searching for signs of shocked material that contained hydrogen, which would indicate that the companion was something other than another carbon-oxygen white dwarf,” she said.

The astronomers found what they were looking for in SN 2015cp, a supernova first detected in 2015. When Hubble made its observations in 2017, it picked up the ultraviolet glow of debris at least 62 billion miles (100 billion kilometers) away from the supernova source. Follow-up observations confirmed the presence of hydrogen.

Based on the survey’s results, the astronomers estimate that no more than 6 percent of Type Ia supernovae involve a messy red-giant companion. Graham said repeated observations of such supernovae should firm up those estimates.

In addition to Graham, the authors of “Delayed Circumstellar Interaction for Type Ia SN 2015cp Revealed by an HST Ultraviolet Imaging Survey” include Chelsea Harris, Peter Nugent, Kate Maguire, Mark Sullivan, Mathew Smith, Stefano Valenti, Ariel Goobar, Ori Fox, Ken Shen, Tom Brink, Alex Filippenko, Patrick Kelly and Curtis McCully. The research was funded by the National Science Foundation, NASA, the European Research Council and the U.K.’s Science and Technology Facilities Council.

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