Division of Theoretical Astronomy, National Astronomical Observatory of Japan

Research Highlights

Discovery of a supernova beyond the standard explosion paradigm

An international research team discovered a supernova that cannot be explained by the current standard theory of supernova explosions. The supernova, named OGLE-2014-SN-074 (OGLE14-073 hereafter), was more than 10 times more energetic than supernovae of a similar kind. OGLE14-073 could be a new kind of supernovae whose existence has been predicted theoretically but has never been observed. This research was published online in Nature Astronomy on September 18, 2017.

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"Discovery of a supernova beyond the standard explosion paradigm"

Figure: OGLE14-073 observed on September 24, 2014. The image was taken with the ESO Faint Object Spectrograph (EFOSC) on the NTT telescope operated by the European Sourthern Observatory (ESO). (Credit: Terreran et al. (2017) Nature Astronomy)

"Hydrogen-rich supernovae beyond the neutrino-driven core-collapse paradigm", Terreran et al., 2017, Nature Astronomy [Nature Astronomy]

Takashi Moriya [personal webpage]

Astronomers Follow Gravitational Waves to Treasure

The research group J-GEM has performed optical and near-infrared follow-up observations of the gravitational wave source GW170817, which was observed with Advanced LIGO and Advanced Virgo on August 17, 2017. The group has successfully detected the optical and near-infrared counterpart of GW170817 and followed the time evolution of its brightness. GW170817 is the first case of gravitational wave detection from a neutron star merger. The observed properties of the counterpart of GW170817 can be accounted for by a "kilonova", which is light powered by the radioactive decay of elements newly-synthesized in a neutron star merger. These results strongly suggest that heavy elements such as gold and platinum are being synthesized via the "r-process" in the neutron star merger GW170817.
The Subaru Telescope in Hawai`i, 1.4-m IRSF telescope in South Africa, 1.8-m MOA-II telescope in New Zealand, and some other Japanese telescopes joined the follow-up observations. The ATERUI supercomputer was used for the theoretical research of "kilonovae." The important findings in "multi-messenger astronomy" benefited from the collaboration between the gravitational wave and optical/infrared observations, and the simulations.

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Subaru Telescope Press Release: "Optical/Infrared Telescopes Follow Gravitational Waves to Treasure"
CfCA Press Release: "Astronomers Follow Gravitational Waves to Treasure"

Figure: Three-color false-color composite images showing the time evolution of the optical and near-infrared counterpart of GW170817 made using data from the Subaru Telescope and IRSF. (Credit: NAOJ/Nagoya University)

“J-GEM observations of an electromagnetic counterpart to the neutron star merger GW170817”, Utsumi et al., 2017, PASJ [PASJ]
“Kilonova from post-merger ejecta as an optical and near-Infrared counterpart of GW170817”, Tanaka et al., 2017, PASJ [PASJ]
“Subaru Hyper Suprime-Cam Survey for An Optical Counterpart of GW170817”, Tominaga et al., 2017, submitted to PASJ [ADS]

Masaomi Tanaka [personal webpage]

Surface Helium Detonation Spells End for White Dwarf

An international team of researchers has found evidence that the brightest stellar explosions in our Universe could be triggered by helium nuclear detonation near the surface of a white dwarf star. Using Hyper Suprime-Cam mounted on the Subaru Telescope, the team detected a type Ia supernova within a day after the explosion, and explained its behavior through a model calculated using the supercomputer ATERUI. This result was reported in Nature published on Oct. 5.
See the full story:
Subaru Telescope Press Release: "Surface Helium Detonation Spells End for White Dwarf"
CfCA Press Release: "Surface Helium Detonation Spells End for White Dwarf"

Figure: A type Ia supernova detected within a day after exploding. Taken with Hyper Suprime-Cam mounted on the Subaru Telescope. (Credit: The University of Tokyo / NAOJ)

"A hybrid type la supernova with an early flash triggered by helium-shell detonation"
Jiang et al., 2017, Nature 550, 80–83 [Nature]
Masaomi Tanaka [personal webpage]

The effect of collective neutrino oscillations on vp process nucleosynthesis

In core collapse supernovae, large numbers of neutrinos are emitted from the protoneutron star after core bounce. At such high neutrino number densities, neutrino-neutrino coherent scatterings cause a non-linear phenomenon called "collective neutrino oscillation" which results in the dramatic flavor transition in neutrino and antineutrino spectra. There was no quantitative study which took into account the influence of collective neutrino oscillations on the vp-process nucleosynthesis consistently. In this work, we study the impact of collective neutrino oscillations on the vp process nucleosynthesis by combining realistic three flavor multiangle simulations with nucleosynthesis network calculations for the first time. We find that the abundances of p-nuclei which are synthesized in the vp process are enhanced by oscillation effects by 10-10^4 times in normal mass hierarchy. Our results imply the necessity of collective neutrino oscillations for the precise nucleosynthesis in neutrino-driven winds and also help understand the origin of solar-system isotopic abundances of molybdenum 92,94 and ruthenium 96,98.

Figure: The abundances of p-nuclei inside neutrino-driven winds which are normalized by solar isotopic abundances. In normal mass hierarchy (red point), the abundances of p-nuclei are enhanced by collective neutrino oscillations compared with no oscillation case (black point).

"Possible effects of collective neutrino oscillations in three-flavor multiangle simulations of supernova vp processes"
Hirokazu Sasaki, Toshitaka Kajino, Tomoya Takiwaki, Takehito Hayakawa, Baha Balantekin, and Yamac Pehlivan
2017, Physical Review D, 96, 043013 [PRD] [arXiv]
Hirokazu Sasaki