Astronomers identify culprit behind binary system’s supernova explosion

An international team of astronomers, including researchers from the University of Southampton, has identified a companion star that triggered a distant supernova by studying debris from the cosmic explosion.

The research collaboration investigated ultraviolet light gathered by the Hubble Space Telescope to deduce that the binary system’s type Ia supernova was likely caused by a white dwarf interacting with a much larger, red giant star.

Southampton researchers Professor Mark Sullivan and Dr Mathew Smith were part of the team of over a dozen scientists that observed the SN 2015cp supernova, some 545 million light years from earth. Almost two years after the supernova’s initial detection, the collaboration picked up the ultraviolet glow of debris around 62 billion miles from the supernova source which showed signs of hydrogen, indicating something other than a common carbon-oxygen white dwarf.

Mathew, who was at the Very Large Telescope (VLT) in Chile when the team approved the finding, explains: “This discovery is the first confirmation that some of these supernovae are formed from binary systems comprised of a white dwarf and larger companion star, perhaps not dissimilar to our own Sun. Understanding these systems is the key to maximising our knowledge of the eventual fate of the Universe, by understanding the nature of dark energy, and this study is a major step forward along that route.”

The team made the finding as part of a wider study into Type Ia supernovae, which occur when a white dwarf star suddenly explodes due to the activity of a binary companion. The explosions are very bright - about five to 10 billion times brighter than the Sun – and can be seen at very great distances. The similar properties of white dwarf stars prior to and during these explosions allows them to be used as ‘standard candles’ that can be studied to measure distances in the universe.

The Hubble Space Telescope was essential for detecting the ultraviolet signature of SN 2015cp, before researchers undertook additional observations from sites including the W. M. Keck Observatory in Hawaii, the Karl G. Jansky Very Large Array in New Mexico, NASA’s Neil Gehrels Swift Observatory space telescope and the European Southern Observatory’s VLT in Chile.

“We examined many supernova systems and only found one such event,” Mark says. “This allows us to estimate that this only occurs in at most one in 20 supernova explosions. It's very exciting because it means these systems do exist and adds further evidence to the diversity of the types of binary systems that can explode as type Ia supernovae.”