SN2016iet, a supernova like no other
– News of August 27, 2019 –
The core of the stars is the place of a perpetual struggle between the radiative pressure of the thermonuclear reactions that grow outwards and the gravity that seeks to bring everything back to the center of the star. The equilibrium point is called the hydrostatic equilibrium. In the most massive stars, collisions between gamma rays and atomic nuclei lead to the creation of electron-positron pairs that annihilate rapidly to form new gamma rays that support the radiative pressure of the core. It’s a fragile balance that can be broken.
Some instabilities can lead to an overproduction of electron-positron pairs. This phenomenon causes a pressure drop in the core of the star and therefore a partial collapse. This results in a violent rise in temperatures and a sudden acceleration of thermonuclear reactions. The resulting explosion dislocates the entire star without leaving anything behind.
It is probably this type of supernova that was observed by GAIA in November 2016. The explosion was called SN2016iet and was the subject of an article on August 15, 2019 in the journal The Astrophysical Journal. SN2016iet is a strange event for more than one reason.
The star that generated this supernova had to start life with a mass of 200 solar masses, that is to say near the limit of what current models allow for the formation of a star. Yet, the explosion took place far from the heart of the host galaxy, where one would expect to find the most massive stars.
In reality, not one but two explosions were observed 100 days apart. The second explosion is probably due to the shock wave that passed through the material previously expelled by the star. But again the timing does not really stick. In order to observe a second peak of luminosity, it means that this matter was still quite close to the star, as if it had been expelled only a few years ago. But we would rather expect a star of this type to slowly get rid of its mass over several millennia.
SN2016iet is a complex event that will be able to feed the research on pair-instability supernovas. The team behind the article will now use the Hubble Space Telescope to try and see what’s left of the supernova. It may be an opportunity to learn a little more about these gigantic stars and their end of life.
N6946-BH1, story of a failed supernova
– News of March 24, 2019
N6946-BH1 is a star that had a rather singular end. It was a very big star, about 25 times more massive than the sun. It has been observed in the NGC 6946 galaxy located about 22 million light-years away from us. We can think that such a massive star ends its life in supernova. Indeed, in 2009 the luminosity of N6946-BH1 suddenly increased to a million times brighter than the sun. It lasted a few months before the star disappeared completely. It may seem very bright but in reality it’s too little for a supernova.
N6946-BH1 is thought to have missed its supernova explosion. It would have collapsed directly into a black hole without going through an explosive phase. While searching the infrared area with the Spitzer Space Telescope, a small shape was observed which is probably due to the black hole accretion disk that formed. But we do not yet know how often this type of death of a star occurs.
The team that followed the death of N6946-BH1 estimates that 10% to 30% of massive stars could dead as failed supernovas. This would perfectly explain why we observe fewer supernovas than we would expect given the massive number of stars. These failed supernovas could also be phenomena that give rise to black holes of several tens of solar masses such as those observed in gravitational waves by LIGO.
Typically, a supernova expels most of the outer layers of the star, thus reducing the size of the central black hole. In the case of N6946-BH1, more of the mass of the star is found in the final black hole, which allows to form more massive black holes. We will probably improve our knowledge of this phenomenon by observing other failed supernovae.
The essentials about supernovas
Core-collapse supernovas are some of the most violent events in the universe. When a massive star reaches the end of its life, it merges heavier and heavier elements until it can not do it anymore. The hydrostatic equilibrium of the star is then broken and gravity regains the upper hand. In a few milliseconds, the core of the star collapses and blows the outer layers in a gigantic explosion.
These events leave behind the collapsed core of the star a neutron star or in the most extreme cases a black hole. This is how the life of the stars, which make at least eight solar masses, ends. But for the most gigantic stars, of more than 130 solar masses, we suppose that a different mechanism comes into play. We speak then of a pair-instability supernova.
Picture by L. Calçada, ESOSources