‘Hiccuping’ stars caught in action in world first
A giant ‘hiccuping’ star has been documented for the first time in the final stages of its life before exploding in a spectacular supernova by an international team of astronomers including from Queen’s University Belfast.
The rare cosmic event has been predicted in theory but never seen before – partly because it happens so infrequently and even then, only to exceptionally large stars, between 60-150 times the mass of our Sun.
Massive stars play a critical role in the Universe; they produce the heavy chemical elements we need to build life, but there are still gaps in knowledge of how they evolve and die. Confirming ‘hiccupping’ stars is a major step in understanding how outsized stars work and how they have shaped our Universe.
An article published in Astrophysical Journal details evidence of this phenomenon, which astrophysicists call ‘Pulsational Pair Instability’ (PPI), whereby massive stars develop very hot cores which contract and expand in rapid succession in the final years, or even days, of their lives.
Colliding Shells
Each time these stars pulsate, they eject a shell of material, gradually stripping down the core star. Sometimes, the ejected shells collide into each other, creating intense bursts of light in our skies. These collisions are the unique signature of PPI.
Astronomers have never previously been able to confirm this observationally, however, because the shell collisions are much fainter than the final supernova explosion, unless the timing between pulses is just right – which is exactly what happened to one star a few years ago.
Lead author Dr Charlotte Angus from the Astrophysical Research Centre (ARC) at Queen’s University Belfast takes up the story:
“In December 2020, we identified a new bright supernova, now named ‘SN2020acct’, in a nearby spiral galaxy called NGC 2981. The light from SN2020acct disappeared pretty quickly.
“But then in February 2021, we saw light coming from the same region of the galaxy again. This is very unusual as supernovae normally don’t reappear!”
The team used telescopes around the world – in Hawaii, Chile, South Africa and the US – to track the ‘supernova’ and found that the first time it appeared, its light was being produced by relatively slow-moving shells of material colliding near the star. In other words, it wasn’t technically a supernova at all. Dr Angus comments:
"When it appeared for a second time, however, it looked very different – now it was expanding much faster, suggesting the core of the star had exploded, marking the end of its life. This was the actual supernova explosion, suggesting that the first flare may have been the elusive PPI."
The astrophysicists then used modelling to confirm that this particular event was indeed an example of PPI. They were able to prove that the two flares of SN2020acct came from a star that started its life weighing around 150 times the mass of the Sun, which underwent a series of extreme pulses in the final 50 days before it exploded. All the conditions were right for it to eject shells close together in time, explaining why the first flare SN2020acct was so bright and got mistaken for a supernova. Dr Angus adds:
“This is the first time that we have ever obtained observations of a PPI candidate during the shell collisions, allowing us to confirm for the first time that this is really happening. That the data matches the modelling predictions is incredibly exciting!”
Collaborators included astronomers from the University of California Santa Cruz and from three recent surveys - the Young Supernova Experiment, the ATLAS survey and the PanSTARRS survey.
Co-author on the paper, Dr Matt Nicholl from ARC at Queen’s University Belfast adds:
“There’s still a lot we don’t know about massive stars but confirming Pulsational Pair Instability as a real thing is a major step forward in knowledge and greater understanding of these amazing cosmic events.”
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Inquiries to Una Bradley u.bradley@qub.ac.uk