Historically, there are two main types of supernovae. One is a thermonuclear supernova-the explosion of a white dwarf star after acquiring matter in a binary system. These white dwarfs are dense gray cores left by low-mass stars (stars whose mass is not more than 8 times that of the sun) when they reach the end of their lives. Another main type of supernova is the core collapse supernova, that is, a massive star-about 10 times the mass of the sun-runs out of nuclear fuel, and its core collapses to form a black hole or neutron star. Electron capture supernovae are located at the boundary of these two types of supernovae. These stars stop nuclear fusion when their inner cores are composed of oxygen, neon and magnesium; Their quality is not enough to make iron.
In the "electron-trapping supernova", some electrons in the oxygen-neon-magnesium core are knocked into their nuclei, a process called electron trapping. This removal of electrons causes the core of a star to bend and collapse under its own weight, which leads to the appearance of electron-trapped supernovae. If the star is a little heavier, the core elements may fuse to produce heavier elements and prolong life. Therefore, this is an "anti-Goldilocks effect": the star is not light enough to escape the fate of the collapse of its core, nor heavy enough to prolong its life, and then dies in different ways.
This is the theory put forward by Kenichi Nomoto of the University of Tokyo in 65438-0980. For decades, theorists have been predicting supernovae looking for electron capture and their SAGB star precursors. These stars should have great mass and lost most of their mass before the explosion, and the mass near the "dying" stars should have unusual chemical composition. Then the electron capture supernova should be very weak, with almost no radioactive falling objects and neutron-rich elements in the core.
The new study, published in Nature-Astronomy, was led by Daichi Hiramatsu, a graduate student at the University of California, Santa Barbara (UCSB) and the Bryce Observatory (LCO) in Lasquin. Pingsong is a core member of the global supernova project, which is a worldwide team of scientists, using dozens of telescopes all over the world and above. The research team found that supernova SN 20 18zd has many unusual features, some of which were first seen in supernovae.
This supernova is relatively close to us-only 31100,000 light years away-located in NGC 2 146 galaxy. This enabled the research team to check the archive images taken by the Hubble Space Telescope before the explosion and detect possible precursor stars before the explosion. The observation results are consistent with another SAGB star recently discovered in the Milky Way, but inconsistent with the model of the red giant star, which is the origin of the common core collapse supernova.
In this study, all published supernova data were reviewed, and it was found that although some supernovae have some indicators of predicted electron capture supernova, only SN 20 18zd has all six indicators-an obvious SAGB protozoa, strong mass loss before supernova, unusual stellar chemical composition, weak explosion, a small amount of radioactivity and a neutron-rich core.
"We started by asking,' What is this weirdo?' "Pingsong said. "Then we examined every aspect of SN 20 18zd and realized that all these can be explained by electron capture scheme."
This new discovery also reveals some secrets of the most famous supernova in the past. In A.D. 1054, a supernova explosion occurred in the Milky Way. According to the records of China and Japan, it is very bright, which can be seen for 23 days during the day and nearly two years at night. The resulting residue, the crab nebula, has been studied in detail. It was once the best candidate for "electron capture supernova", but this is uncertain, partly because the explosion occurred nearly 1000 years ago. The new results increase people's confidence that SN 1054 is an electron-trapped supernova in history. It also explains why the supernova is relatively bright compared with the model: its brightness may be artificially enhanced by the collision between the supernova ejecta and the material thrown by the protostar, as shown in SN 20 18zd.
Dr Ken Nomoto of the University of Tokyo at IPMU is very excited that his theory has been confirmed. He added: "I'm glad that the electron capture supernova has finally been discovered. My colleagues and I predicted its existence 40 years ago, and it is related to the Crab Nebula. I am very grateful for the great efforts made to obtain these observations. This is a wonderful case of combining observation with theory. "
Pingsong added: "For all of us, this is a' Eureka moment' and we can contribute to closing the 40-year theoretical cycle. For me personally, because my astronomy career began when I looked at amazing pictures of the universe in the high school library, one of which was the iconic crab nebula photographed by the Hubble Space Telescope. "
"When we discover a new astrophysical object, the word Rosetta Stone is often used as a metaphor," said Dr. Andrew Howell, a staff member of the Brece Observatory in Lasquin and a part-time teacher at UCLA. "But in this case, I think it is appropriate. This supernova is actually helping us to interpret the Millennium cultural records around the world. It is helping us to connect something that we don't fully understand, namely the Crab Nebula, with another thing that we have an incredible modern record, namely this supernova. In this process, it is teaching us basic physical knowledge: how some neutron stars are produced, how extreme stars live and die, and how our constituent elements are produced and dispersed in the universe. " Dr Howell is the head of the global supernova project and the doctoral supervisor of the main author.