7 mins read 29 Jun 2021

Supernova Companions the Key to Binary Stellar Evolution

Supernovae often occur in multiple-star systems and when they do, the fallout can damage the companion stars, causing them to become bigger and brighter. Now, astronomers have worked out that measuring how long they take to heal can provide insight into how massive stars evolve.

Artist's impression of a supernova. A core-collapse supernova happens when the pressure from the heat generated by the star's core is insufficient to prevent the star's collapse due to self-gravity. Credit: James Josephides / Swinburne University of Technology

Sometimes when a star reaches the end of its life it does so with a cataclysmic bang. In quick time it shreds itself, throwing out the raw materials that will seed the next generation of stars and planets. Even companion stars can be caught up in the explosion and scarred by the fallout.

Astronomers have found that this scarring can tell them a great deal about the binary star system as it was before the explosion even occurred. With most stars being a part of multiple star systems, this is of particular interest to scientists trying to understand their evolution.

But not all stars will go supernova as they run out of fuel – only the more massive ones have enough self-gravity to actually explode. A star needs to have roughly eight times more mass than our Sun for what is known as a core-collapse supernova to occur.

And it is gravity that drives the processes that lead to this dramatic ending.

For most of its life, a star exists in a state known as hydrostatic equilibrium, where the inward and outward forces on the star are finely balanced. Gravity draws in surrounding matter towards the star’s core, while radiation pressure from the heat being generated within pushes outwards and prevents the star from imploding.

In fact, these two forces are inextricably linked. If the core were to cool a little, the inward force of gravity would exceed the outward radiation pressure and the star would contract. The contraction would increase the temperature and pressure of the star again, returning it to equilibrium.

The real excitement comes as the star runs out of fuel and can no longer support itself against its own weight. Within a split second the core collapses, sending a shockwave radiating out through the star blowing it apart and causing one of the most energetic events we see anywhere in the Universe.

How much energy are we talking about? Roughly as much as the Sun will generate over something like 10-billion years. All produced in barely a fraction of that time.

If there is a nearby companion star, it can be hit by debris from the explosion. When this happens, the surface heats up and causes the star to swell, a bit like having a burn blister on your skin.

The star blister can be 10 or even 100 times larger than the star itself, but it lasts only for a very short time. Within a few decades, the blister heals, and the star shrinks back to its original form.

The team of Australian and Japanese astronomers, including post-doc researcher Dr Ryosuke Hirai from Monash University and the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav), have carried out hundreds of computer simulations to investigate how companion stars inflate depending on their interactions with nearby supernovae. They then applied their results to SN2006jc, a supernova that was first seen by amateur astronomers and preceded by something that was, well, a little confusing.

Supernova Imposters

An image of the supernova SN2006jc taken by the Ultaviolet/Optical telescope onboard the Neil Gehrels Swift Observatory. This image was from 2006, but in 2004 it appeared as though the star had gone supernova only to later be labelled as a supernova imposter. Credit: NASA / Swift / S. Immler

In October of 2004, a massive star located in a barred spiral galaxy in the constellation Lynx was observed to be throwing huge amounts of material into space. 

Amateur astronomer Koichi Itagaki had been scanning the skies with a 60-cm telescope from his personal observatory in the forested mountains outside Yamagata City when the event occurred. A prolific supernova hunter known by astronomers around the world, Itagaki initially suspected that he was witnessing the spectacular explosion of another massive star.

As it turned out, this was just the prelude to the main act. This event was later found to have been a supernova imposter, a stellar outburst that looked like a faint supernova but one that only managed to eject a shell of gas (albeit a very large shell of gas) while leaving the original star mostly intact.

Two years later and Itagaki, along with American amateur astronomer Tim Puckett and Italian amateur astronomer Roberto Gorelli, watched as the star finally gave up the inexorable struggle to hold itself up against its own self-gravity and blew itself apart. Material from the explosion took only a few weeks to catch up to the previously ejected shell that had been travelling away from the star for two years.

The supernova was called SN2006jc, reported in October 2006 and located in the barred spiral galaxy UGC 4904, located some 77 million light-years away in the constellation Lynx.

Supernova imposters typically appear as bright as the brightest stars and look for all intents and purposes like a true core-collapse supernova. Some researchers are not even convinced about which are real and which are the imposters.

While true supernovae are the result of gravity winning over radiation pressure, the imposters seem to hit a point in their evolution where the outward forces dominate. But really, astronomers still don’t what mechanism in the stars causes some of them, and not others, to erupt so violently.

The star Betelgeuse has had a lot of press lately, what with its unexpected brightening and dimming, causing many people to become excited by the possibility that it could go supernova sometime soon (though, 'soon' in astronomical terms is within the next 100,000 or so years). Additionally, one of the southern hemisphere stargazer’s favourites - Eta Carinae - has already had a supernova imposter episode of its own some 170-years ago and could end its life at any time. At the time its magnitude increased to become the second brightest star in the sky, before fading away over several years. 

Blistered Stars

The larger of the two stars in Eta Carinae is notoriously unstable and has undergone supernova imposter events. It is one of the closest stars to Earth that is likely to explode in a supernova in the relatively near future. Credit: ESA/NASA

But for this study, Dr Hirai and his collaborators wanted to apply their results to SN2006jc, the supernova first picked up by those expert amateur astronomers.

The handful of supernovae companion stars that have actually been observed, like the one associated with SN2006jc, have been found to have unusually low temperatures. When they are struck by ejected material and swell up, they become brighter, but their outer layers cool as they expand.

Computer simulations have shown that their newfound brightness depends only on the mass of the star, not on the strength of the interaction with the supernova. As well, the duration of the blistering or swelling gives an indication of how close the two companions were before the event and the energy of the supernova explosion.

If SN2006jc is an inflated star, as the research team believes, it should shrink back to its normal size about 15 years after the supernova event. That is, any time now, astronomers should observe the star dimming again and appearing hotter. Getting telescope time at the big observatories is a very competitive process, but it seems as though this would be a star worth keeping tabs on over the next few years.

The correlations found in this research will help astronomers to better understand the evolution of binary star systems where one of the stars goes supernova, but to take full advantage they will need to find more companion stars.

If we can do that, it will help us to understand a bit more about the lives of stars. Let the hunt begin.