A hybrid type Ia supernova with an early flash triggered by helium-shell detonation

• Published in: Nature (London) 2017-10, Vol.550 (7674), p.80-83
• Main Authors: Jiang, Ji-an, Doi, Mamoru, Maeda, Keiichi, Shigeyama, Toshikazu, Nomoto, Ken’ichi, Yasuda, Naoki, Jha, Saurabh W., Tanaka, Masaomi, Morokuma, Tomoki, Tominaga, Nozomu, Ivezić, Željko, Ruiz-Lapuente, Pilar, Stritzinger, Maximilian D., Mazzali, Paolo A., Ashall, Christopher, Mould, Jeremy, Baade, Dietrich, Suzuki, Nao, Connolly, Andrew J., Patat, Ferdinando, Wang, Lifan, Yoachim, Peter, Jones, David, Furusawa, Hisanori, Miyazaki, Satoshi
• Format: Article
• Language: English
• Subjects: 639/33/34; 639/33/34/864; Carbon; Companion stars; Computer simulation; Diffraction; Dwarf stars; Ejecta; Ejection; Explosions; Helium; Humanities and Social Sciences; letter; Light; Light curve; multidisciplinary; Observations; Oxygen; Science; Supernova; Supernovae; Supernovas; Thermonuclear explosions; White dwarf stars; White dwarfs
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/nature23908

The detection and simulation of a type Ia supernova with an early, red flash suggests that it formed through detonation of the helium shell of a white dwarf, rather than by collision of the ejecta with a companion star or by merging with another white dwarf. A different kind of supernova Type Ia supernovae have rather uniform and normalizable light curves, making them suitable for cosmology, yet there remains uncertainty over what paths lead to the explosion. Several years ago a claim was made that a flash seen soon after the explosion was evidence of the shock wave hitting a normal companion star, although most other evidence so far suggests that the explosions arise from the merger of two white dwarfs. Ji-an Jiang and collaborators report observations of a red flash half a day after a type Ia explosion. Their observations lead them to the conclusion that the flash came from the detonation of a thin helium shell surrounding the exploding star. The authors conclude that their finding supports the existence of the previously proposed helium-ignition pathway. Type Ia supernovae arise from the thermonuclear explosion of white-dwarf stars that have cores of carbon and oxygen 1 , 2 . The uniformity of their light curves makes these supernovae powerful cosmological distance indicators 3 , 4 , but there have long been debates about exactly how their explosion is triggered and what kind of companion stars are involved 2 , 5 , 6 . For example, the recent detection of the early ultraviolet pulse of a peculiar, subluminous type Ia supernova has been claimed as evidence for an interaction between a red-giant or a main-sequence companion and ejecta from a white-dwarf explosion 7 , 8 . Here we report observations of a prominent but red optical flash that appears about half a day after the explosion of a type Ia supernova. This supernova shows hybrid features of different supernova subclasses, namely a light curve that is typical of normal-brightness supernovae, but with strong titanium absorption, which is commonly seen in the spectra of subluminous ones. We argue that this early flash does not occur through previously suggested mechanisms such as the companion–ejecta interaction 8 , 9 , 10 . Instead, our simulations show that it could occur through detonation of a thin helium shell either on a near-Chandrasekhar-mass white dwarf, or on a sub-Chandrasekhar-mass white dwarf merging with a less-massive white dwarf. Our finding provides evidence that one branch of previous