Astronomers spot a new type of supernova
ASTRONOMERS HAVE LONG KNOWN that stars more massive than about eight Suns die in powerful blasts, known as type II or core-collapse supernovae. But in 1980, a paper in Publications of the Astronomical Society of Japan suggested that stars in a specific mass range — eight to 10 times the mass of the Sun — would die in a unique explosion called an electron-capture supernova. Prior to such a star’s death, its core is supported by electron degeneracy pressure, generated in dense environments where physics prevents free electrons from getting too close. But as magnesium and neon atoms in the core absorb (i.e., capture) these free-floating electrons, the pressure drops and the star’s inner regions collapse into a neutron star as the outer regions rebound as a supernova.
While some supernovae have exhibited hints they could be an electron-capture supernova, none exactly matched the theory. Until March 2018, that is, when Japanese amateur astronomer Koichi Itagaki spotted a supernova in the galaxy NGC 2146, which lies about 31 million light-years away. Researchers raced to image the explosion, called SN 2018zd, with the Hubble Space Telescope. Comparing their data with previous Hubble images of the galaxy, they identified the progenitor star. And after fully analyzing the star and its explosion, the team published a paper June 28 in Nature Astronomy announcing it neatly fits all six expected criteria for an electron-capture supernova.
“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,” said the study’s lead author, Daichi Hiramatsu at the University of California, Santa Barbara, and Las Cumbres Observatory, in a press release. “It was such a ‘eureka moment’ for all of us that we can contribute to closing the 40-year-old theoretical loop.”
The progenitor was the specific mass and type of red giant theorized to produce an electron-capture supernova, and the density and composition of its stellar winds prior to its demise also matched expectations. The explosion itself — its light lingering as the shock wave hit material the star had sloughed off before its death — behaved exactly as astronomers modeled for this type of stellar blast. Finally, the particular chemical fingerprints of the debris matched calculations of what an electron-capture supernova leaves behind.
Now, researchers can slot this information into the bigger picture of how stars live and die. The masses of stars that create electron-capture supernovae fit neatly between stars that leave behind a white dwarf, like the Sun, and those that explode in a core-collapse supernova. “One of the main questions in astronomy is to compare how stars evolve and how they die,” said study co-author Stefano Valenti at the
University of California, Davis. “There are many links still missing, so this is very exciting.”