Astronomers Identified a Star that made its Partner go Supernova

SN 2015cp, a supernova at a distance of 545 million light-years, was triggered by a companion star which marked the death of its partner in the binary system.   

Supernovae are quite common because all the stars of the universe eventually run out of fuel and die with a massive explosion. Having said that, there is another reason which could lead to a supernova. In the past, astronomers suggested that if a close companion star disturbs its partner too much, it can explode despite having the fuel to carry out nuclear fusion. Binary star systems are the most suitable places for such events as a couple of stars are striving to share dominion. These claims lacked physical evidence because a lot of effort is needed to trace the companion star, which initiated the supernova.

Recently, an international team of astronomers announced that SN 2015cp was produced by such an event. They performed repeated observations of the supernova and found hydrogen-rich debris that the companion star might have shed before the explosion. Melissa Graham, the Lead Author of the paper who is an astronomer from the University of Washington, mentioned that the material from the supernova smashed into this debris at 10% the speed of light, causing it to glow. Consequently, a bright ultraviolet light was observed by a lot of observatories including the Hubble Space Telescope.

The researching team made this discovery while studying a particular type of supernova called Type Ia. This type of supernova occurs when a Carbon-Oxygen white dwarf suddenly explodes due to the activity of a binary companion. Despite the fact that these stars are collapsed cores of larger stars, they are so stable that they can persist for billions of years. Type Ia supernovae have been used for a number of cosmological studies because of their consistent luminosity. For instance, they have been used as indirect evidence for the existence of dark energy. Similarly, they helped scientists to estimate the expansion rate of the universe. Despite all that, researchers are not confident about the kinds of companion stars that lead to a Type Ia event. Graham acknowledged that by saying,

“All of the science to date that has been done using Type Ia supernovae, including research on dark energy and the expansion of the universe, rests on the assumption that we know reasonably well what these ‘cosmic lighthouses’ are and how they work. 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.”

Most of the times, the companion star in a Type Ia supernova is a Carbon-Oxygen white dwarf, which eliminates the possibility of Hydrogen-rich debris. Scientists have found only a small fraction of Type Ia supernovae that have impacted with Hydrogen-rich material in the long run. Previously, at least two of these supernovae showed signs of glowing debris months after the explosion but this wasn’t enough to determine that different kinds of companion stars can produce a Type Ia supernova. For this reason, the researching team used the Hubble Space Telescope to observe ultraviolet emissions from 70 different Type Ia supernovae over a period of 1-3 years after the initial explosion. Graham commented on that in the following words:

“By looking years after the initial event, 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.”

SN 2015cp was first detected in 2015. 686 days after the explosion, Hubble detected an ultraviolet glow of debris, which was at least 100 billion kilometers away from the source of the supernova. The researchers compared SN 2015cp to other Type Ia supernovae and found that no more than 6% of them have such a companion. Graham elaborated that they performed a lot of detailed observations of this supernova before arriving at these estimates which shows the complexity of the job. She said,

“The discovery and follow-up of SN 2015cp’s emission really demonstrates how it takes many astronomers, and a wide variety of types of telescopes, working together to understand transient cosmic phenomena. It is also a perfect example of the role of serendipity in astronomical studies: If Hubble had looked at SN 2015cp just a month or two later, we wouldn’t have seen anything.”