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The Forefront of Space Science

AKARI's Mid-IR All-Sky Survey
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Feature of mid-infrared all-sky survey data - the case of Tycho's supernova remnant

The mid-infrared all-sky survey by AKARI has already produced many new results including: insight into the site where stars are formed in chain reaction; and observation of debris disks close to central stars, which is key to elucidate a scenario of planet formation. The survey data is also being analyzed for extended objects, not just point sources. Of those achievements, I would like to introduce one recent topic below.

Stars more than eight times heavier than the Sun cause a huge explosion at their end of life (i.e., supernova explosion) and release elements synthesized inside the stars to outer space. "Tycho's supernova remnant" is a remnant of a supernova explosion observed by the Danish astronomer Tycho Brahe in the 16th century. It is located relatively close (5,000 to 10,000 light years away) and one of the valuable supernova remnants recorded in human history.

The left picture of Fig 3 shows a color composite image of the remnant. Blue shows the distribution of expanding high-temperature plasma due to the explosion, which was observed by Japanese X-ray astronomical satellite SUZUKA. Most materials released from the supernova are expanding in the upper-right direction. Green shows a map of radiowave carbon monoxide (12CO) emission lines, indicating the distribution of molecular clouds originally existing in outer space. From this picture, it appears likely that the supernova is expanding freely in the upper-right direction while colliding with interstellar materials in the upper-left direction. In fact, expansion speed at the shock-wave plane in the upper left is lower than in the upper right. We believe that the speed is slowed by the collision with interstellar materials.

Figure 3
Figure 3. Left: Three-color composite image of Tycho's supernova remnant. Blue is X-ray image (0.2 - 10kev) by SUZAKU. Green is radiowave 12CO emission lines. Red is 18µm band image by AKARI.
Right: Comparison of the position of mid-infrared (18µm band) radiation, and the positions of shock wave plane (white line) and discontinuous plane (green line)

Red shows the observation results by mid-infrared (18µm band), which measures radiation from the heated, warm (~100K) dust mainly existing in outer space. The heat source of the dust is thought to be shock waves between the expanding materials of the supernova and interstellar materials. Although the dust shines in the shell shape generally, brighter areas in mid-infrared are seen in upper-left and upper-right areas particularly. According to AKARI's far-infrared observation, we already know that massive clouds of cool dust originating from interstellar space exist together with molecular gases in the upper-left direction of the remnant. Therefore, we can assume that the upper-left bright area shows the event where dust originally existing in interstellar space is warmed at the shock-wave plane of the supernova. Meanwhile there are no molecular gases and cool dust originating from interstellar space in the upper right. The warm particles emitting mid-infrared there are likely to emerge suddenly in empty space.

The right picture of Fig 3 shows the especially bright areas in mid-infrared and offers a detailed comparison of the positions of the supernova's (1) shock-wave plane (the tip of the blast) and (2) discontinuous plane (border plane between the swept interstellar materials and the materials released from supernova). They were previously imaged by the X-ray observation. The upper-left warm dust shines in between the shock-wave plane (white line) and the discontinuous plane (green line). This supports the theory that the swept materials originating from interstellar space are heated. We found, however, that the upper-right warm dust differed from the theory. This dust shone inside the discontinuous plane, i.e., inside the materials released from the supernova. We can interpret this as follows: the high-temperature materials ejected from the supernova are cooled, condense and eventually become solid particles.

This discovery is valuable because it reveals the site where solid particles are formed in a supernova remnant, which is important for research on the material cycle in our current galactic system and the early universe. Observations by the next-generation infrared satellite SPICA will allow us to examine observationally and in more detail the supply of formed particles to interstellar space and, further, the reduction of matter to form the next stars and planets.

Our research has been conducted jointly with the X-ray astronomical group of Nagoya University. By predicating the physical state and origin of the solid particles in interstellar space, AKARI's infrared data would be a great tool to elucidate a variety of physical phenomena in the universe and also applicable to issues discussed in the X-ray and radiowave research fields.


Following a long period of development and operation, AKARI is now in the harvest stage in terms of science. We believe that new findings will continue to emerge and expect more active cooperation with other-wavelength observational astronomers and theoretical astronomers. AKARI is a JAXA project and being promoted jointly with the European Space Agency (ESA), University of Tokyo, Nagoya University, Seoul National University, and universities in Europe. I would like to express my appreciation to all those involved on AKARI.

Daisuke Ishihara

*South Atlantic Anomaly (SAA)
The Van Allen Belt, present at an altitude of 1,000km, descends to an altitude of 300 to 400 km over Brazil. Satellites in low-earth orbit are exposed to massive radiation when they pass through it.

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