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People knew a phenomena that a very bright star suddenly appears in the sky for thousands of years (and there are the actual historical records of that!). It is called Supernova, and now we know it is a result of the stellar explosion. This explosion in the Universe spreads out the elements created by nuclear fusion in the progenitor star or the explosion; the supernovae, therefore, play important roles in terms of the history of the cosmic nucleosynthesis. However, we still do not understand much about the mechanisms of the explosion. In this lab, we are studying about explosions, progenitor star, and nucleosynthesis by observing “Supernova Remnants (SNRs)” which are the high-temperature plasma structure remained thousands of years after the explosion with Suzaku and other X-ray satellites. (Photo: ©NASA/CXC)
SNRs observed by Chandra X-ray Observatory
Neutron Stars are born after a gravitational collapse of a massive star and believed that their degenerate pressure by neutrons counteracts the force of their gravity. The radius is very small (～10 km) while the mass is close to Sun. Therefore, the density reaches one billion ton per 1 cm^3. Nowadays, ～2000 neutron stars have been observed in our galaxy and their rotation periods are mainly 0.1 – 10 s. Moreover, the magnetic field strength of most neutron stars are one handred million tesla and some sources, called Magnetar, are considered to have extremely strong magnetic field in the 10 billion tesla . Therefore, neutron stars are thought to be the most strong magnet in the universe. Currently, we study the plasma phenomena around the neutron stars by cosmic X-ray observation. (Photo : © NASA/Dana Berry)
Active Galactic Nuclei
Every galaxy in the universe harbors, at its center, a supermassive black hole with a mass of million–billions Solar mass. When a lot of materials are swallowed into a supermassive black hole, their gravitational energy is efficiently converted to radiative energy which is comparable or even larger than the sum of lights from all stars included in a galaxy. These objects are called Active Galactic Nuclei (AGNs), and many astrophysicists study them by observing in various wavelengths. In our laboratory, we mainly observe X-rays from AGNs, to examine structures of time and space around a black hole, physical properties of materials just before swallowed into a black hole, and feedback to galaxies from AGNs.
Stars that are lighter than eight times of our Sun’s mass shine by consuming its internal fuel hydrogen via nuclear fusion for about a few hunred million to one billion years. When the fuel runs out, an external layer of the star is expelled outward sometimes leaving a beautiful planetary nebula, and the dense core of the star is left as a white dwarf star of which diameter is about 10,000 km and weighes a half of the Solar mass (corresponding the size of the Earth, weighing 150 thouthand times heavier). About 10% of the Galaxy’s stellar mass is account by White dwarf stars, and they are believed to be the synthesizer of some part of heavy metals in the universe via supernova explosion which takes place when gas accretes onto a white dwarf from a companion star in a binary system, or when a white dwarf star collides with its companion white dwarf star in a peculiar binary system. Some members of Tamagawa Group are studying X-ray spectra of white dwarf stars to measure their masses precisely using Suzaku and ASTRO-H satellites.
(photo: ©NASA, ESA, HEIC, and The Hubble Heritage Team, STScI, AURA)
Planetary nebulae shoot by the Hubble space telescope
Habitable Space Environments of Jupiter and Saturn
The moons at Jupiter, Europa and Ganymede, potentially have the liquid water ocean underneath the icy surface, where the extra-terrestrial life could exist. The moon Io has volcanoes which are the most active in our solar system bodies. The volcanic gases are provided into the space environments around Jupiter called ‘magnetosphere’ inside which magnetic fields of Jupiter and Io’s volcanic gases are enclosed. The volcanic gases were rapid ionized and picked up by Jupiter’s magnetic fields, which is 20,000 times stronger than Earth, starting co-rotation with Jupiter at a period of rotation (~10 hours). The corotating plasma blows the moons. The dynamics of the plasma is essential for the icy habitable moons. Saturn’s magnetosphere has similar characteristics. The moon Enceladus was confirmed to have the liquid water plume providing water-related plasma into the magnetosphere.
In the space environments at Jupiter and Saturn, there are excited the intense auroral emission which is 100 times stronger than Earth and the plasma heating of which temperature reaches up to 1 million degree as the solar corona. They are still big problems because the drivers of these phenomena are completely unknown. It is unclear which the corotating plasma and magnetic field around Jupiter give them sufficient energy, and/or the variability of the external solar wind provides the energy. We address this problem based on the multiple wavelength remote sensing simultaneously using radio, ultraviolet, and X-ray emissions which give us the plasma dynamics at different energies.
Photo : Artist’s illustration of Jupiter and its moons. Ganymede (upper right), Europa (bottom left), Calisto (bottom right), Io (middle left), Jupiter (right side of Io), their structure, and surrounding space environments are described. (©ESA、JUICE Red book)