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“Shocking” findings from a supernova illuminate the stellar culprits

Supernova 2017cbv, on the outskirts of the spiral galaxy NGC 5643. Data are from the Las Cumbres Observatory Global Supernova Project and the Carnegie-Irvine Galaxy Survey. Credit: BJ Fulton, LCO.

Astronomers using Las Cumbres Observatory (LCO) have observed an exploding star slamming into a nearby companion star and detected the fleeting blue glow from the interaction at an unprecedented level of detail. The observations reveal the surprising identity of the mysterious companion star, a feat made possible by recent advances in linking telescopes into a robotic network.

The supernova in question is SN 2017cbv, a thermonuclear (Type Ia) supernova, the type astronomers use to measure the acceleration of the expansion of the universe. Type Ia supernovae are known to be the explosions of white dwarfs, though they need to gain mass from a companion star to explode. The identity of this companion has been hotly debated for more than 50 years.

Griffin Hosseinzadeh, a graduate student at the University of California, Santa Barbara, led the study, soon to be published in The Astrophysical Journal Letters. He said, “We’ve been looking for this effect — a supernova crashing into its companion star — since it was predicted in 2010. Hints have been seen before, but this time the evidence is overwhelming. The data are beautiful!”

White dwarf stars are the dead cores of what used to be normal stars like the sun. The prevailing theory over the last few years is that the supernovae happen when two white dwarfs spiral together and merge.

In contrast, this study infers that the white dwarf was stealing matter from a much larger companion star, approximately 20 times the radius of the sun. This caused the white dwarf to explode; and the collision of the supernova with the companion star shocked the supernova material, heating it to a blue glow that was heavy in ultraviolet light. Such a shock could not have been produced if the companion were another white dwarf star.

Excess ultraviolet light from Supernova 2017cbv in the few days after explosion attributed to a collision between the ejected material and a binary companion star. This short-lived phenomenon can only be clearly revealed with observations taken several times per day — a feat made possible by Las Cumbres Observatory’s global network of telescopes.

“We’ve seen a supernova obliterating planets in science fiction, as happened in 2009’s STAR TREK. That’s nothing! They can wreck nearby stars too, releasing unbelievable amounts of energy in the process. The universe is crazier than science fiction authors have dared to imagine,” said Andy Howell, a staff scientist at LCO, and Hosseinzadeh’s PhD advisor. Howell is also adjunct faculty at the University of California, Santa Barbara.

David Sand, an associate professor at the University of Arizona, discovered the supernova on March 10, 2017, in the galaxy NGC 5643. At only 55 million lightyears away, it was one of the closest supernovae discovered in recent years. SN 2017cbv was found by the DLT40 survey, using the PROMPT telescope in Chile, which monitors galaxies nightly at Distances Less Than 40 Megaparsecs (120 million lightyears). This was one of the earliest catches ever — within a day, perhaps even hours, of its explosion. The DLT40 survey was created by Sand and Stefano Valenti, an assistant professor at the University of California, Davis. Both were previously postdoctoral researchers at LCO.

Within minutes of discovery, Sand activated observations with LCO’s global network of 18 robotic telescopes. They are spaced around the Earth so that there is always one on the night side. This allowed the team to take immediate and near-continuous observations. “With Las Cumbres Observatory’s ability to monitor the supernova every few hours, we were able to see the full extent of the rise and fall of the blue glow for the first time,” said Hosseinzadeh. “Conventional telescopes would have had only a data point or two and missed it.”

“This is like getting astronomical superpowers that allow you to see the universe in new ways,” said Howell. “And every time that happens, we find something shocking! This is an amazing result thanks to the hard work of engineers, programmers, and scientists at Las Cumbres Observatory over many years,” continued Howell. “These capabilities were just a dream a few years ago. But now we’re living the dream, and unlocking the origins of supernovae in the process!”

Simulation of a Type Ia supernova in which material ejected from the explosion (red) runs into a companion star (blue). This collision adds extra ultraviolet light to the supernova as viewed from Earth. Credit: Dan Kasen.

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