Astronomers confirm that there is a third type of supernova explosion

Supernovae occur when stars explode. When you think of a supernova, the type you are probably imagining is a type II or core-collapse supernova. This type of cosmic explosion occurs when a star about 10 times the mass of our Sun (or more) explodes at the end of its life, leaving behind a neutron star or black hole. The other type of supernova, type I, occurs when the remnant of a Sun-like star, called a white dwarf, siphons material from a close companion. Matter accumulates on the surface of the white dwarf, and once it exceeds a certain mass limit, an exciting thermonuclear explosion tears the white dwarf apart.

However, calculations dating back to 1980 show that there should be a the third type of supernova, called electron capture supernova. This type of explosion only occurs for stars in a narrow mass range – 8 to 10 solar masses – that straddle the line between silent evolution into white dwarfs and the explosive birth of neutron stars or black holes. when they die.

Electron capture supernovae also produce neutron stars like some type II supernovae. But before the star can die, the magnesium and neon atoms that have built up in its core begin to capture the electrons floating around them, which are responsible for the external pressure that holds the star’s core together. stable. As the electrons are absorbed, this reduces this outward pressure, causing the inner regions of the star to collapse into a neutron star while the outer regions simultaneously explode like a supernova explosion.

In March 2018, Japanese amateur astronomer Koichi Itagaki spotted a new supernova in the galaxy NGC 2146, located about 30 to 40 million light years away in the constellation Camelopardalis. Now the researchers have analyzed the explosion and, in an article published on June 28 in Nature astronomy, announced that it perfectly matches the profile of an electron-capturing supernova.

A perfect match

What’s special about this new supernova, called SN 2018zd, is that astronomers were able to compare Hubble and Spitzer space telescope images of its host galaxy before and after the explosion. This helped them identify the likely progenitor star that precipitated the explosion.

“It was one of the key components that had never been made for other candidate supernovae for electron capture – they had never had a viable identified progenitor star, the exploding star,” Study co-author Alex Filippenko of the University of California, Berkeley said in a press release. This time, however, astronomers were able to compare both the star and the light from its supernova explosion to the expected profile of an electron-capturing supernova.

These observations correspond perfectly to the expectations, corresponding to the six criteria expected for such an event.

First, the ancestor was a type of red giant, or aging star, called a super-asymptotic giant branch star. These stars have between 8 and 10 solar masses and are said to be the progenitors of electron-capturing supernovae. Second, this ancestor had lost much of his mass before exploding, blowing his outer layers in a cloud of matter around him. Third, this material showed the unique chemical makeup expected before an electron-capturing supernova: abundant helium, carbon, and nitrogen, but little oxygen. Fourth, the explosion itself was weaker than one would expect for a core collapsed supernova. Fifth, the light from the explosion behaved as astronomers expected for an electron-capturing supernova: the light persisted for more than 100 days as the shock wave material struck them. outer layers that the star had previously destroyed, generating a deposit glow. Finally, the makeup of the material left behind – particularly the presence of stable nickel but no radioactive nickel (the latter being common after nucleus collapse) – is also what astronomers expect from an electron-capturing supernova.

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