The star that burns twice as bright

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An helium explosion heralds the end of a dying star

An international astronomical team led by Ji-an Jiang (graduate student of the University of Tokyo), Mamoru Doi (U. Tokyo), Keiichi Maeda (Kyoto University) and Toshikazu Shigeyama (U. Tokyo), called “the MUSSES team”, has found evidence that the brightest stellar explosions in our universe are triggered by a helium nuclear explosion at the surface of a white dwarf.

The conclusion was reached by examining observational data of a type Ia supernova discovered within a day of the explosion by the MUSSES project. The team used the wide-field camera “Hyper Suprime-Cam” mounted on the 8.2-m Subaru telescope, and simulating light curves and spectra based on various explosion scenarios using the supercomputer ATERUI. This study was published in Nature on Oct. 5, 2017.

Type Ia Supernovae are believed to be an explosion of a white dwarf. Because of the uniform and extremely high brightness (about 5 billion times brighter than our Sun), they are widely used as the standard candle for measuring distance in astronomy, leading to the discovery of the accelerating expansion of the universe (Nobel prize in physics 2011). However, no one has been able to find solid evidence as to what triggers these explosions.

The MUSSUS project is designed to find supernovae just after the explosion. “Among more than 100 supernovae discovered within a single night, we finally identified one very infant type Ia supernova caught by our observations. Surprisingly, the supernova in the first few days was much brighter than expected,” explained Jiang. A new question was then evident to the researchers – what makes this flash and how is it related to the explosion mechanism?

The team further conducted intensive computational simulations based on different kinds of theoretical models. They were finally able to identify one scenario, which they thought was the only possible solution. In this scenario, the accumulation of helium (He) on the surface of the white dwarf first ignites, and explosive He burning eventually leads to disruption of the whole star. The bright flash is then generated by the energy released from the unstable nucleosynthesis products of the burning He.

"Once we noticed the possibility of this He detonation scenario, all of the observational data of this supernova, the evolution of brightness, color, and spectral features, became readily explainable," says Maeda. "This is the first robust evidence that one theoretically predicted stellar explosion mechanism, proposed in the early 1980's, does truly exist in our universe. Our next question is if such a mechanism is unique or common among type Ia supernovae."

The MUSSES team will carry out further investigations to find more supernovae within a day after explosion and test their models. "We are expecting to find many more of these supernovae, which should bring us further insight into their mechanisms. A more precise understanding of their history and behavior will help all researchers to perform more accurate cosmological measurement," concludes Doi.

Astronomical art of a pre-explosion white dwarf triggered by the helium explosion at its surface. The nuclear explosion of the surface helium layer triggers inward shock waves, and the central carbon nuclear fusion is subsequently ignited. (Credit: Institute of Astronomy, the University of Tokyo)

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Ji-an Jiang, Mamoru Doi, Keiichi Maeda, Toshikazu Shigeyama, Ken'ichi Nomoto, Naoki Yasuda, Saurabh W. Jha, Masaomi Tanaka, Tomoki Morokuma, Nozomu Tominaga, Željko Ivezić, Pilar Ruiz-Lapuente, Maximilian D. Stritzinger, Paolo A. Mazzali, Christopher Ashall, Jeremy Mould, Dietrich Baade, Nao Suzuki, Andrew J. Connolly, Ferdinando Patat, Lifan Wang, Peter Yoachim, David Jones, Hisanori Furusawa & Satoshi Miyazaki (2017). A hybrid type Ia supernova with an early flash triggered by helium-shell detonation. Nature, 550, 80–83.