The Goldilocks Supernova: The discovery of a new type of supernova illuminates a medieval mystery
A worldwide team led by UC Santa Barbara scientists at Las Cumbres Observatory has discovered the first convincing evidence for a new type of stellar explosion — an electron-capture supernova. While they have been theorized for 40 years, real-world examples have been elusive. They are thought to arise from the explosions of massive super-asymptotic giant branch (SAGB) stars, for which there has also been scant evidence. The discovery, published in Nature Astronomy, also sheds new light on the thousand-year mystery of the supernova from A.D. 1054 that was visible all over the world in the daytime, before eventually becoming the Crab Nebula.
Historically, supernovae have fallen into two main types: thermonuclear and iron-core collapse. A thermonuclear supernova is the explosion of a white dwarf star after it gains matter in a binary star system. These white dwarfs are the dense cores of ash that remain after a low-mass star (one up to about 8 times the mass of the sun) reaches the end of its life. An iron core-collapse supernova occurs when a massive star — one more than about 10 times the mass of the sun — runs out of nuclear fuel and its iron core collapses, creating a black hole or neutron star. Between these two main types of supernovae are electron-capture supernovae. These stars stop fusion when their cores are made of oxygen, neon and magnesium; they aren’t massive enough to create iron.
While gravity is always trying to crush a star, what keeps most stars from collapsing is either ongoing fusion or, in cores where fusion has stopped, the fact that you can’t pack the atoms any tighter. In an electron capture supernova, some of the electrons in the oxygen-neon-magnesium core get smashed into their atomic nuclei in a process called electron capture. This removal of electrons causes the core of the star to buckle under its own weight and collapse, resulting in an electron-capture supernova.
If the star had been slightly heavier, the core elements could have fused to create heavier elements, prolonging its life. So it is a kind of reverse Goldilocks situation: The star isn’t light enough to escape its core collapsing, nor is it heavy enough to prolong its life and die later via different means.
That’s the theory that was formulated beginning in 1980 by Ken’ichi Nomoto of the University of Tokyo and others. Over the decades, theorists have formulated predictions of what to look for in an electron-capture supernova and their SAGB star progenitors. The stars should have a lot of mass, lose much of it before exploding, and this mass near the dying star should be of an unusual chemical composition. Then the electron-capture supernova should be weak, have little radioactive fallout, and have neutron-rich elements in the core.
The new study is led by Daichi Hiramatsu, a graduate student at UC Santa Barbara and Las Cumbres Observatory (LCO). Hiramatsu is a core member of the Global Supernova Project, a worldwide team of scientists using dozens of telescopes around and above the globe. The team found that the supernova SN 2018zd had many unusual characteristics, some of which were seen for the first time in a supernova.
