2021-07-10

Scientists discover a new type of star explosion: the first convincing evidence of “electron capture supernova”

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Scientists discover a new type of star explosion: the first convincing evidence of “electron capture supernova”

Historically, there are two main types of supernovae. One is the explosion of a thermonuclear supernova, a white dwarf star that acquires matter in a binary star system. These white dwarfs are dense gray cores left by low-mass stars (stars with a mass no more than 8 times the sun) after they reach the end of their lives. Another major type of supernova is the iron core collapse supernova, that is, a massive star-more than about 10 times the mass of the sun-exhausts its nuclear fuel, and its iron core collapses to form a black hole or neutron star. Electron capture supernovae are on the boundary between these two types of supernovae. These stars cease nuclear fusion when their cores are composed of oxygen, neon, and magnesium; their mass is not enough to create iron.

In “electron capture supernova”, some electrons in the oxygen-neon-magnesium core are smashed into their nuclei. This process is called electron capture. This removal of electrons causes the star’s core to bend and collapse under its own weight, leading to the emergence of electron-trapping supernovae. If the star is slightly heavier, the core elements may fuse, producing heavier elements and extending its lifespan. Therefore, this is an “anti-blonde girl effect”: the star is not light enough to escape the fate of its core collapse, nor is it heavy enough to extend its life, and later die through different methods.

This is a theory put forward by Ken’ichi Nomoto of the University of Tokyo and others since 1980. For decades, theorists have worked out predictions to look for in electron capture supernovae and their SAGB protoplasma. These stars should have a lot of mass and lose most of their mass before they explode, and the masses of these near “dying” stars should have an unusual chemical composition. Then the electron capture supernova should be weak, with almost no radioactive fallouts, and with neutron-rich elements in its core.

The new research published in Nature-Astronomy was led by Daichi Hiramatsu, a graduate student at the University of California, Santa Barbara (UCSB) and Las Cumbres Observatory (LCO). Hiramatsu is a core member of the Global Supernova Project, a worldwide team of scientists who use dozens of telescopes all over the world and in the sky. The team discovered that the supernova SN 2018zd has many unusual features, some of which have been seen in supernovae for the first time.

Scientists discover a new type of star explosion: the first convincing evidence of “electron capture supernova”(1)

The supernova is relatively close– Only 31 million light-years away-in the galaxy NGC 2146. This allowed the research team to examine archive images taken by the Hubble Space Telescope before the explosion and detect possible precursor stars before it exploded. The observation results are consistent with another SAGB star recently discovered in the Milky Way, but inconsistent with the model of the red superstar, which is the origin of the ordinary iron-core collapsed supernova.

The study reviewed all published supernova data and found that although some supernovae have some indicators of predicted electron capture supernovae, only SN 2018zd has all six indicators-an obvious SAGB protist, Strong pre-supernova mass loss, unusual stellar chemical composition, weak explosion, small amount of radioactivity, and neutron-rich core.

“We asked’what is this weirdo’ from the beginning?” Hiramatsu said. “Then we checked every aspect of SN 2018zd and realized that all of these can be explained in the electron capture scheme.”

The new discoveries also illuminate some of the mysteries of the most famous supernovas of the past. Place. In 1054 AD, a supernova occurred in the Milky Way. According to Chinese and Japanese records, it is so bright that it can be seen for 23 days during the day and nearly two years at night. The resulting remnant, the Crab Nebula, has been studied in great detail. It used to be the best candidate for “electron capture supernova”, but this is uncertain, partly because the explosion occurred nearly a thousand years ago. The new results increase people’s confidence that the historical SN 1054 is an electron-captured supernova. It also explains why the supernova is relatively bright compared to the model: its brightness may have been artificially enhanced by the collision of the supernova ejection with the material ejected by the native star, as seen in SN 2018zd.

Dr. Ken Nomoto of Kavli IPMU of the University of Tokyo is excited that his theory has been confirmed. He added: “I am very happy that the electron capture supernova has finally been discovered. My colleagues and I It has been predicted before, and it is related to the Crab Nebula. I am very grateful for the tremendous efforts made to obtain these observations. This is a wonderful case of combining observation and theory.”

Hiramatsu added: “For all of us, this is an’Eureka moment’, and we can contribute to closing the 40-year theoretical cycle. For me personally, because my astronomy career is based on my The high school library started by looking at amazing pictures of the universe. One of them was the iconic Crab Nebula taken by the Hubble Space Telescope.”

“When we discovered a new astrophysics When it comes to objects, the term Rosetta Stone is often used as a metaphor,” said Dr. Andrew Howell, a staff member of the Las Cumbres Observatory and a part-time teacher at the University of California, Los Angeles, “but in this case, I think it Very appropriate. This supernova is actually helping us to interpret the thousand-year record of cultures from all over the world. It is helping us to understand one thing we don’t fully understand, namely the Crab Nebula, which is different from our incredible modern record. One thing is that this supernova is connected. In the process, it is teaching us basic physics: how some neutron stars are produced, how extreme stars survive and die, and the elements that we are composed of How is it produced and dispersed in the universe.” Dr. Howell is the head of the global supernova project and the doctoral supervisor of the lead author.