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A Double Detonation Supernova

Jan 10, 2024

Artist’s conception of a white dwarf star accreting helium from disk.  When the helium builds up on the surface, it causes an explosion, which triggers a second detonation in the core of the star, exploding it.

Image Credit: Institute of Astronomy, University of Tokyo

Rare Insight into the Progenitors of Type Ia Supernovae

Scientists at Las Cumbres Observatory have uncovered the origins of an unusual Type Ia supernova, giving rare insight into what makes them explode. Type Ia supernovae are nature’s best distance indicators and have been used to map the expansion of the universe, revealing the presence of Dark Energy. Even still, key details of their origin, like what size star explodes and how it is triggered, have remained elusive for nearly a century.

Estefania Padilla Gonzalez, a graduate student at the University of California Santa Barbara and a researcher at LCO, is the lead investigator of "SN 2022joj: A Potential Double Detonation with a Thin Helium Shell'' which is available on Archiv.

On May 8, 2022, the Zwicky Transient Facility at the Palomar Observatory discovered the supernova SN 2022joj. Following the discovery, the Global Supernova Project began observing the event using the Las Cumbres Observatory network of telescopes, and combined their data with information from other facilities.

These observations showed that this supernova has some peculiar properties. The light emitted a few days after the explosion was exceedingly red in color and evolved over time to become bluer like a normal Type Ia supernova. By fitting computer simulations of supernova explosions to the data the team determined that the best theoretical explanation of SN 2022joj is a double detonation – the supernova exploded twice.

It has long been known that Type Ia supernovae are the explosions of white dwarf stars. A white dwarf is the burned-out core of a star that was once like the Sun but has run out of fuel – it is a leftover ball of carbon and oxygen ash, only about the size of the Earth. If left alone, a white dwarf cannot make energy or explode. But if it is in a binary system with another star, it can steal matter from its companion. If it gains enough mass, a white dwarf can become a bomb and explode.

For years the leading model for a Type Ia supernova has been that they are the result of a white dwarf near the Chandrasekhar mass – 1.4 times the mass of our Sun. This is a fundamental limit set by quantum mechanics. As the white dwarf approaches the limit, carbon is ignited in the core, exploding the star.

The new study is at odds with this old idea. It reveals that instead the supernova was the explosion of a white dwarf only about the mass of the Sun, nowhere near the limit. The explosion was triggered by the buildup of helium on the surface of the white dwarf, stolen from its companion star. Helium under these conditions is unstable and detonates, driving a shock into the core of the star, which causes a second detonation, exploding it.

Astronomers have had computer models of these “double detonations” for decades, but they didn’t fit the observations of most Type Ia supernovae. Initially, theorists thought that the helium shell had to be a thick one, as much as 15-20% of the mass of the sun to trigger a detonation. These models produced very red explosions. In recent years a few supernovae have been found that matched these thick helium shell models. But since they didn’t look like normal Type Ia supernovae, they were not thought to be their progenitors.

SN 2022joj is a kind of “missing link” connecting the exotic thick helium shell detonations to more normal Type Ia supernovae. According to simulations, it was ignited by a thin helium shell – one only about 2% of the mass of the sun. That ignited helium is what gives the supernova its early red color. The thinness of the shell meant that the supernova quickly lost its red color, and began to look like a normal Type Ia supernova after a few days. Most supernovae were not observed as early as SN 2022joj, so it is possible these signatures are there in many Type Ia but have been missed. It is also possible that more normal supernovae are ignited by even thinner helium shells.

Dr. Andy Howell, Senior Scientist at LCO and Adjunct Professor at UCSB, is the PhD advisor of Ms. Padilla Gonzalez, and a coauthor on the study. Dr. Howell said, “Until now, I thought helium shell detonations were just exotic weirdos. But this supernova shows that helium shell detonations could be the triggering mechanism for most, or maybe even all Type Ia supernovae. I’ve been trying to determine the progenitors of Type Ia supernovae for decades, and this is one of the most exciting breakthroughs I’ve ever seen.”

Estefania Padilla Gonzalez presented her findings in a press conference at the American Astronomical Society meeting in New Orleans on January 10.

Ms. Padilla Gonzalez speaking at the AAS 243 conference in New Orleans on January 10.