A global team led by scientists from UC Santa Barbara at the Las Cumbres Observatory have discovered the first compelling evidence of a new type of stellar explosion – an electron-capturing supernova. Although they’ve been theorized for 40 years, real-world examples have been elusive. They are thought to result from explosions of super-asymptotic giant-branched massive stars (SAGBs), for which there is also little evidence. The discovery, published in Nature astronomy, also sheds new light on the millennial mystery of the 1054 AD supernova that was visible all over the world during the day, before eventually becoming the Crab Nebula.
Historically, supernovae have fallen into two main types: thermonuclear collapse and iron core collapse. A thermonuclear supernova is the explosion of a white dwarf star after gaining matter in a binary star system. These white dwarfs are the dense ash cores 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 collapsing supernova occurs when a massive star – a star more than 10 times the mass of the sun – runs out of nuclear fuel and its iron core collapses, creating a black hole or star in the sun. neutrons. Between these two main types of supernovae are electron capture supernovae. These stars stop fusion when their nuclei are made up of oxygen, neon and magnesium; they are not massive enough to create iron.
While gravity is still trying to crush a star, what keeps most stars from collapsing is either the fusion in progress or, in the nuclei where the fusion has stopped, the fact that you cannot wrap the atoms more tightly. In an electron-capturing supernova, some of the electrons in the oxygen-neon-magnesium nucleus are crushed in their atomic nuclei in a process called electron capture. This removal of electrons causes the star’s core to deform under its own weight and collapse, resulting in an electron-capturing supernova.
Had the star been slightly heavier, the core elements could have coalesced to create heavier elements, extending its lifespan. So it’s sort of a reverse Goldilocks situation: the star is not light enough to escape its core collapse, nor heavy enough to extend its life and later die by other means. .
This is the theory that was formulated from 1980 by Ken’ichi Nomoto of the University of Tokyo and others. Over the decades, theorists have made predictions about what to look for in an electron-capturing supernova and their SAGB star progenitors. The stars should have a lot of mass, lose a lot before they explode, and this mass near the dying star should be of an unusual chemical composition. Then the electron-capturing supernova should be weak, have little radioactive fallout, and have neutron-rich elements in the nucleus.
The new study is being led by Daichi Hiramatsu, a graduate student from UC Santa Barbara and Las Cumbres Observatory (LCO). Hiramatsu is a vital member of the Global Supernova Project, a global team of scientists using dozens of telescopes around and above the globe. The team discovered that the SN 2018zd supernova had many unusual features, some of which were first seen in a supernova.
This helped the supernova to be relatively close – just 31 million light years away – in the galaxy NGC 2146. This allowed the team to examine archival images taken by the Hubble Space Telescope before. the explosion and detect the probable progenitor star before it. exploded. The observations were consistent with another recently identified SAGB star in the Milky Way, but inconsistent with models of red supergiants, the progenitors of normal supernovae with collapsed iron nucleus.
The authors examined all of the published data on supernovae and found that while some had some of the predicted indicators for electron-capture supernovae, only SN 2018zd had all six: an apparent progenitor of SAGB, a large loss. pre-supernova mass, an unusual star. chemical composition, a weak explosion, little radioactivity and a core rich in neutrons.
“We started off by asking” what the hell is this weirdo? “Then we looked at all aspects of SN 2018zd and realized that all of them could be explained in the electron capture scenario. “
The new findings also shed light on some mysteries of the most famous supernova of the past. In 1054 AD, a supernova occurred in the Milky Way which, according to Chinese and Japanese records, was so bright it could be seen during the day for 23 days and at night for almost two years. The resulting remnant, the Crab Nebula, has been studied in detail.
The Crab Nebula was previously the best candidate for an electron-capturing supernova, but its status was uncertain in part because the explosion occurred nearly a thousand years ago. The new result increases confidence that the historic SN 1054 was an electron-capture supernova. It also explains why this supernova was relatively bright compared to the models: its luminosity was probably artificially increased by the supernova ejecta colliding with the material rejected by the progenitor star, as seen in SN 2018zd.
Ken Nomoto of IPMU Kavli at the University of Tokyo expressed his enthusiasm for his theory to be confirmed. “I am very happy that the electron capture supernova has finally been discovered, that my colleagues and I predicted that it exists and has a connection with the Crab Nebula 40 years ago,” he said. declared. “I very much appreciate the tremendous effort that went into obtaining these observations. It’s a wonderful case of combining observations and theory.
Hiramatsu added, “It was such a ‘eureka moment’ for all of us that we can help close the 40-year-old theoretical loop, and for me personally because my career in astronomy started when I watched the awesome movies. images of the Universe in the high school library, one of which was the iconic Crab Nebula taken by the Hubble Space Telescope.
“The term Rosetta Stone is too often used as an analogy when we find a new astrophysical object,” said Andrew Howell, scientist at Las Cumbres Observatory and assistant professor at UCSB, “but in this case, I think it is appropriate. This supernova literally helps us decode the millennial archives of cultures around the world. And it helps us associate one thing that we don’t fully understand, the Crab Nebula, with another thing that we have incredible modern records of, this supernova. In the process, he teaches us basic physics: how certain neutron stars are made, how extreme stars live and die, and how the elements we are made of are created and scattered throughout the universe. Howell is also the leader of the Global Supernova project and the PhD of lead author Hiramatsu. advise.
Reference: June 28, 2021, Nature astronomy.
DOI: 10.1038 / s41550-021-01384-2