This finding supports a clumpy 3-D scenario of supernova explosions rather than the widely accepted bipolar explosion scenario. It advances our understanding of how supernovae explode, a process that has been a persistent mystery.
The mystery of supernovae
Stars heavier than eight solar masses will end their lives with a brilliant explosion called a supernova. A supernova ejects elements synthesized within its star that are heavier than hydrogen and helium, the main elements of the primeval universe. The ejection of these heavier elements into interstellar space has enriched the chemical composition of the universe.
Despite its important role in the evolution of the universe, the process of how supernova explosions occur has been unclear. Based on recent numerical simulations, researchers agree that supernovae would not succeed as one-dimensional, spherical events and that multidimensional effects are important for understanding their occurrence. Scientists have proposed two main scenarios to explain how supernova explosions occur: (1) a bipolar explosion facilitated by rotation, and (2) a clumpy 3-D explosion driven by convection. However, scientists have not known which scenario is more plausible because they have not actually observed the shape of supernovae.
Seeing the “shape” of supernovae by polarization
Although it would seem easy to see the shapes of supernovae by simply taking a picture of them, observing them is a challenging task. Because most supernovae occur in galaxies millions or hundreds of millions of light-years away, they only look like a point, even though they expand at a speed of 6,200 miles (10,000 kilometers) per second.
The current research team used a special method of detection to reveal the shape of supernovae: They measured so-called “polarization,” which supplies information about the direction of vibrating electromagnetic waves. They performed numerical simulations for emissions from supernovae and found clearly different polarization patterns for bipolar and clumpy explosions. An object shows various angles of polarization in a clumpy explosion while it shows a single angle of polarization in a bipolar explosion.
Based on hypotheses derived from their simulations, the group used the FOCAS to conduct polarimetric observations of nearby supernovae; such observations measure the intensity and direction of polarization. Because the researchers did not know when the supernovae would appear, they could not assign an observing time in advance. Fortunately, Subaru Telescope has a “Target of Opportunity” mode that overcomes this difficulty and enables a dynamic allocation of the observing time. Thanks to this mode, the team succeeded in conducting polarimetric observations of two so-called “stripped-envelope supernovae” (SN 2009mi and SN 2009jf), which do not have hydrogen surrounding them and are the best targets for studying explosion geometry.
Revealing the 3-D structure
The team detected the polarization from the two supernovae, which clearly indicated that the supernovae are not usually round. They also found that each supernova had various angles of polarization, a finding consistent with the scenario of a clumpy 3-D explosion.
When the team added the two new supernovae to the ones from previous observations, they had a total of six stripped envelope supernovae, five of which showed the signature of clumpy 3-D geometry. The research showed that the clumpy 3-D shape is common in supernovae. Although the bipolar explosion scenario is widely accepted, the findings of this research support the clumpy 3-D scenario of supernova explosions. Convective motion in the explosions could account for this clumpy shape. This result serves as a catalyst to further understand how supernova explosions occur.