UCSB scientists likely prove existence of rare electron-capture star explosion
A worldwide team led by UCSB scientists recently found convincing evidence of a stellar explosion roughly 31 million light-years away.
And what they found just might be the origin of the Crab Nebula, shedding light on a thousand-year mystery from A.D. 1054.
The researchers found the first solid evidence that “electron-capture supernovae” exist. These explosions occur when some electrons in the oxygen-neon-magnesium core get smashed into their atomic nuclei, causing the core of the star to buckle under its own weight and collapse.
The study was led by a graduate student at UCSB and Las Cumbres Observatory, Daichi Hiramatsu, who is also a core member of the Global Supernova Project. Andrew Howell, a staff scientist at Las Cumbres Observatory and adjunct faculty at UCSB, was the advisor for Mr. Hiramatsu as well.
“It was such an exciting moment for me to close the long-standing theoretical loop that was formulated even before I was born,” Mr. Hiramatsu told the News-Press. “The process was rather gradual, but certainly rewarding, like putting together a thousand-piece jigsaw puzzle.”
Up until now, scientists knew of two supernova types: thermonuclear and iron core collapse, Mr. Hiramatsu explained.
Thermonuclear supernovae originate from white dwarfs (stars that had roughly less than eight times the mass of the sun at birth) while iron core collapse supernovae originate from massive stars (roughly more than 10 times the mass of the sun at birth).
Thermonuclear supernovae occur when the stars gain enough matter from their companion to reach a certain mass for a thermonuclear explosion.
Iron core collapses occur when the massive stars stop nuclear fusions once they form iron cores, losing the support against gravity and resulting in a core collapse.
Somewhere in the middle, the scientists found, are electron-capture supernovae, specifically considering their star mass, which is roughly eight to 10 times the mass of the sun.
“The stars in the intermediate mass range are not heavy enough to fuse up to iron in their core. Instead, they stop nuclear fusions when they form an oxygen+neon+magnesium core that is pressure supported by electrons,” Mr. Hiramatsu said. “When the core becomes dense and hot enough, the so-called electron-capture reactions take place, in which neon and magnesium eat up electrons, converting their protons into neutrons.”
This results in the core collapsing under its own gravity like the iron core collapse explosion.
But it’s triggered by the electron-capture reactions rather than thermonuclear ignition.
The supernova the graduate student and his team studied — SN 2018zd — revealed six peculiar elements providing the solid evidence of the theory: 1) a faint progenitor star; 2) dense gas surrounding the progenitor star; 3) enriched chemical composition of the gas; 4) low explosion energy; 5) large brightness that dropped roughly four months after the explosion; and 6) neutron-rich nucleosynthesis in the explosion.
Mr. Hiramatsu said all of the elements “can be naturally explained in the electron-capture scenario.”
The Crab Nebula, too, has many similar elements that can be naturally explained in the electron-capture scenario, he said, including: dense surrounding gas, peculiar chemical composition, low ejecta mass, low kinetic energy and neutron-rich nucleosynthesis.
However, the scientist said this doesn’t quite confirm its origin — yet.
“Since the supernova happened nearly a thousand years ago, however, the information on the progenitor star and supernova explosion themselves is missing, so some uncertainties still remain,” Mr. Hiramatsu said. “We are hoping that this discovery can bridge our understanding of electron-capture supernovae from early explosion (SN 2018zd) to late nebula (the Crab Nebula) phases.”
When asked why it took four decades for scientists to find this convincing evidence of the unique origin theory of the Crab Nebula, the researcher said he’s been asking the same question for a while.
He has come up with three main reasons.
One is that electron-capture supernovae are “intrinsically rare” — scientists estimate their rate to be roughly a few percent or even less of all core-collapse supernovae.
Second, the explosion energy of these specific supernovae is lower than the others, so they could be fainter and harder to discover, Mr. Hiramatsu said.
Lastly, he said discovering this required extensive observations to check the criteria, so it’s possible that some supernovae were overlooked as normal iron core collapse in nature.
Mr. Hiramatsu and his team were able to make their discovery by using the worldwide telescope network of Las Cumbres Observatory. Specifically, they used the 40cm and 1m telescopes in Texas and the 2m telescope in Hawaii. Some data came from Japanese amateur astronomers as well, along with Bok and MMT telescopes in Arizona, Keck telescopes in Hawaii, and the Hubble and Spitzer Space Telescopes.
Leading the charge to hopefully explain the origin of the Crab Nebula was a full-circle type moment for Mr. Hiramatsu, as he first got interested in astrophysics when he saw pictures of the universe, one of which was the iconic Crab Nebula.
He said this made him realize that “those beautiful pieces can be explained in terms of the math and physics that we know of.”
Even more ironic is that SN 2018zd exploded on Mr. Hiramatsu’s mother’s 60th birthday.
Overall, the graduate student spoke to the significance of his and his team’s efforts.
“This is an important milestone in our understanding of stellar evolution and supernova physics by defining the borderline between stars that explode and stars that don’t, which ultimately has influences on many fields of astrophysics, such as the chemical evolution of the universe.”