Astronomers have discovered a new type of supernova, or star explosion, and it provides a new window into the violent life cycle of stars. The new research, focused on supernova 2018zd, confirms a prediction made by University of Tokyo astronomer Ken'ichi Nomoto more than 40 years ago.
Amateur astronomer Koichi Itagaki in Japan observed supernova 2018zd in March 2018, spurring astronomers to use telescopes to study it about three hours after it occurred. The supernova happened about 31 million light-years from Earth and archival images from the Hubble and Spitzer space telescopes allowed scientists to see the faint star prior to explosion. This was the first time astronomers were able to see a star like this before and after going supernova.
The main support that prevents stars from collapsing beneath the weight of their own gravity is the energy in their core.
Typically, supernovae occur in two flavors. During a core-collapse supernova, a massive star (more than 10 times the mass of our sun) exhausts its fuel and the star's core caves in to a black hole or a dense remnant called a neutron star. The other type is called a thermonuclear supernova, and it occurs when a low-mass star remnant called a white dwarf -- usually less than eight times the mass of our sun -- explodes after pulling matter from a companion star into itself.
But what happens to star between eight and 10 solar masses, such as the star involved in supernova 2018zd? They explode a little differently.
This third, previously unobserved type is referred to as an electron capture supernova -- and it was orginally described by Nomoto in 1980. As the star's core loses fuel, gravitational forces push the core's electrons and fuse them with atomic nuclei. This sudden drop in electron pressure triggers a collapse and the star buckles beneath its own weight. What remains is a dense neutron star with a little more mass than our sun.
A study based on the new research published Monday in the journal Nature Astronomy.
"One of the main questions in astronomy is to compare how stars evolve and how they die," said Stefano Valenti, study coauthor and professor of physics and astronomy at the University of California, Davis, in a statement. "There are many links still missing, so this is very exciting."
Understanding the electron capture supernova
Daichi Hiramatsu, a graduate student at the University of California, Santa Barbara, and Las Cumbres Observatory, led an observation team that gathered data on the 2018zd supernova for two years after it was first observed. The more data they collected, the more the researchers realized this may be the first example of an electron capture supernova.
Nomoto's theory about these supernovae suggests that they carry an unusual chemical signature after occurring, which the researchers observed in 2018zd's data. It also matched the other five criteria in Nomoto's theory required for the proposed supernova type. These include a strong loss of mass prior to the supernova, a weak explosion, small radioactivity, a core rich in elements like oxygen, neon and magnesium, and a Super-Asymptotic Giant Branch type star. These SAGB stars, which are rare, are bloated old red giant stars.
"We started by asking 'what's this weirdo?' Then we examined every aspect of SN 2018zd and realized that all of them can be explained in the electron-capture scenario," Hiramatsu said.
Because these stars exist within a limited mass range, they aren't light enough to prevent their cores from collapsing, but they also aren't heavy enough to create life-prolonging heavier elements, like iron.
"This is the best known case for this interesting category of supernovae that is in between the mass range for the exploding white dwarf and the iron core of a massive star that collapses and then rebounds and leads to an explosion, the so-called core-collapse supernovae," said Alex Filippenko, a professor of astronomy at the University of California, Berkeley, in a statement. "This study significantly increases our understanding of the final stages of stellar evolution."
Filippenko said the fact that the researchers had access to Hubble images showing the star before and after exploding helped them confirm the type of supernova that occurred.
And this type of supernova is likely responsible for a nebula that lit up the skies almost a thousand years ago, according to the researchers.
Creating the Crab Nebula
In 1054, a supernova occurred in our Milky Way galaxy that was so bright, it could be seen in the sky during the daytime around the world for 23 days -- and it remained visible in the night sky for almost two years.
The result of this supernova was the famous Crab Nebula, a point of fascination for astronomers over the years that they now as a result of the new study believe was created by an electron capture supernova.
While the Crab Nebula has long been considered the best known example of electron capture supernova, if it existed, there was some doubt because the event happened so long ago.
The supernova's brightness was likely enhanced by material jettisoned by the explosion colliding with material previously released from the star -- something also witnessed during the 2018zd supernova.
"I am very pleased that the electron-capture supernova was finally discovered, which my colleagues and I predicted to exist and have a connection to the Crab Nebula 40 years ago," Nomoto said in a statement. "I very much appreciate the great efforts involved in obtaining these observations. This is a wonderful case of the combination of observations and theory."
"It was such a 'Eureka moment' for all of us that we can contribute to closing the 40-year-old theoretical loop, and for me personally because my career in astronomy started when I looked at the stunning pictures of the Universe in the high school library, one of which was the iconic Crab Nebula taken by the Hubble Space Telescope," Hiramatsu added.
Astronomers will continue the search to see if they can find more examples of electron capture supernovae.
"The term Rosetta Stone is used too often as an analogy when we find a new astrophysical object, but in this case I think it is fitting," said Andrew Howell, a staff scientist at Las Cumbres Observatory and adjunct faculty at the University of California, Santa Barbara, in a statement.
"This supernova is literally helping us decode thousand-year-old records from cultures all over the world," said Howell who was also involved in the study. "And it is helping us associate one thing we don't fully understand, the Crab Nebula, with another thing we have incredible modern records of, this supernova. In the process it is teaching us about fundamental physics: how some neutron stars get made, how extreme stars live and die, and about how the elements we're made of get created and scattered around the universe."
