An electron-capture supernova, a new type of stellar explosion, was discovered, according to a study published in Nature Astronomy that included Dr. Iair Arcavi of Tel Aviv University, the university announced on Wednesday. While electron-capture supernovae have been theorized about for 40 years, this is the first time that one has been discovered.
A supernova occurs when the forces that shape a star experience a sudden imbalance. Every star is contracted by gravity, and the star needs to counter balance this force. The sun, for example, uses nuclear fusion in its core. The pressure that is produced by the core opposes the gravitational pull until the nuclear fusion eventually dies out, and the core begins to collapse. Different stars have different types of collapses. A white dwarf is what would become of a star like the sun. In the case of a white dwarf, further collapse is prevented by its high density.
The problem for stars ten times the size of the sun is that the gravitational pull is able to surpass their density. In this case, the core continues to collapse until a neutron star or black hole is formed, accompanied by a giant explosion. the star to collapses and explodes due to the removal of the electron quantum force, which is cause by the electrons are capturing onto atomic nuclei. The result is the collapse and explosion of the star otherwise known as a supernova.
Historically, two types of supernovae have been witnessed. The thermonuclear supernova is what occurs when a white dwarf star gains matter in a binary star system. The other, a core-collapse supernova, is what happens when a neutron star or a black hole is created by the core collapse of a star more than ten times the size of the sun.
In the 1980's, Ken'ichi Nomoto of the University of Tokyo theorized that an electron-capture supernova is what would occur in a supernova that was somewhere between a thermonuclear supernova and a core-collapse supernova. Theorists have since tried to predict what this would look like.
The study, which was led by Daichi Hiramatsu of the University of California, now has answers and was published in Nature Astronomy. It focused on supernova SN2018zd from galaxy NGC 2146, which was discovered by Japanese astronomer Koihchi Itagaki in 2018.
The research shows that the supernova has all the properties of an electron-capture supernova. In addition, the researchers were able to see the star in pre-explosion archival images taken by Hubble Space Telescope, and the images show a star that would be expected to produce the electron-capture supernova.
Other supernovae in the past have presented a few of the indicators if the supernova, SN2018zd is the first to have all six – a progenitor star of the mass range, strong pre-supernova mass loss, unusual chemical composition, weak explosion, little radioactivity and neutron-rich material.
"We started by asking 'what's this weirdo?'," said Hiramatsu. "Then we examined every aspect of SN2018zd and realized all of them can be explained in the electron-capture scenario."
Another benefit of this discovery is that it likely explains one of the most famous past supernovae from 1054 CE. The supernova happened in our Milky Way galaxy, and according to records of the time, it was so bright it could be seen in the daytime and cast shadows at night. The remnant of the supernova, the Crab Nebula, has been studied in great detail and found to have an unusual composition. As it happened so long ago, it cannot be said for sure, but it now believed that the supernova could have been an electron-capture.
It's amazing that we can shed light on historical events in the universe with modern instruments," said Dr. Arcavi. "Today, with robotic telescopes that can scan the sky in unprecedented efficiency, we can discover more and more rare events which are critical for understanding the laws of nature, without having to wait 1,000 years between one event and the next." 