Scientists have discovered a supernova that provides an unprecedented look at the first moments of a violent stellar explosion and reveals that the phenomena is even more mysterious than previously thought. The light from the explosion’s first hours showed an unexpected pattern, according to the researchers from the University of Hawaii and Carnegie Institution for Science in the US.
Type Ia supernovae are fundamental to our understanding of the cosmos. Their nuclear furnaces are crucial for generating many of the elements around us, and they are used as cosmic rulers to measure distances across the universe. Despite their importance, the actual mechanism that triggers a Type Ia supernova explosion has remained elusive for decades.
Astronomers have long tried to get detailed data at the initial moments of these explosions, with the hope of figuring out how these phenomena are triggered. This finally happened in February of this year with the discovery of a Type Ia supernova called ASASSN-18bt (also known as SN 2018oh). ASASSN-18bt was discovered by the All-Sky Automated Survey for Supernovae (ASAS-SN), an international network of telescopes that routinely scans the sky for supernovae and other cosmic explosions. NASA’s Kepler space telescope was simultaneously able to take complementary data of this event.
“ASASSN-18bt is the nearest and brightest supernova yet observed by Kepler, so it offered an excellent opportunity to test the predominant theories of supernova formation,” said Ben Shappee, lead author of the study published in The Astrophysical Journal. Combining data from ASAS-SN, Kepler, and telescopes around the world, the astronomers realised that ASASSN-18bt looked unusual during its first couple of days. “Many supernovae show a gradual increase in the light they put out,” said Maria Drout, of University of Toronto. “But for this event, you could clearly see there’s something unusual and exciting happening in the early times – an unexpected additional emission,” said Drout.
Type Ia supernovae originate from the thermonuclear explosion of a white dwarf star – the dead core left over by a Sun-like star after it exhausts its nuclear fuel. Material must be added to the white dwarf from a companion star to trigger the explosion, but the nature of the companion star and how the fuel is transferred has long been debated.
One possibility is that this additional light seen during the supernova’s early times could be from the exploding white dwarf colliding with the companion star.
Although this was the initial hypothesis, detailed comparisons with Piro’s theoretical modeling work demonstrated that this additional light may have a different, unexplained origin. “While the steep increase in ASASSN-18bt’s early brightness could indicate that the explosion collides with another star, our follow-up data don’t fit predictions for how this should look,” said Tom Holoien, from Carnegie. “Other possibilities, such as an unusual distribution of radioactive material in the exploded star, are a better explanation for what we saw,” he said.