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The Forefront of Space Science

Lifetime of Interstellar Dust Explored by AKARI Satellite
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We observed supernova 2006jc, which was discovered by amateur astronomer Kouichi Itagaki in Oct. 2006. The infrared astronomical satellite AKARI observed it about a half year after its explosion. The visible-light observation conducted continuously after its supernova explosion with the Subaru Telescope of the National Astronomical Observatory of Japan (NAOJ) and the Kanata Telescope of Hiroshima University revealed that the supernova began to darken two months after its explosion, and a half year later, it had become so dark as to be barely observable by the Subaru Telescope. On the other hand, AKARI observed the supernova emitting bright infrared a half year after the explosion. These results showed that the dust produced around the supernova caused bright thermal radiation. We believe that this was the moment of the dustís birth around the supernova.

We investigated the nature of the infrared radiation from the dust in more detail using near- to mid-infrared photometric data and near-infrared spectral data from AKARI, and near-infrared photometric data of the same period from the MAGNUM telescope of the University of Tokyo. As a result, two constituents were discovered: carbonaceous dust at approx. 500 deg. C (the high-temperature constituent) responsible for thermal emission in the near-infrared region; and carbonaceous dust at approx. 50 deg. C (the low-temperature constituent) responsible for mid-infrared excess emission over the thermal emission carried by the high-temperature constituent. From the comparison study with theoretical calculation on dust formation, we reached the conclusion that the high-temperature constituent is new born dust from gases released by the supernova explosion. Meanwhile the low-temperature constituent is existing dust, that had been produced by the material released before the explosion, and had surrounded the former parent star having caused the explosion. In addition, by comparing our observation results with the latest star-evolution model, we found that the parent star of supernova 2006jc had a mass of around 40 times the Sunís at the time of its birth, but its mass at the time of the explosion decreased to about seven times the Solar due to continual intensive mass release.

AKARI was fortunate to witness the brief event lasting only half a year at the end of a star-evolution process that had taken several million years. It provided us with valuable information on the birth of interstellar dust and the evolution of a high-mass stars. It is still uncertain, however, if this process is generally applicable to the evolution of any large-mass star. Nonetheless, at least the study result brought us the important suggestion that large mass stars with tens of times the mass of the Sun can contribute to the formation of interstellar dust, not only at the time of their supernova explosion but also throughout their entire evolution.



Figure 2
Figure 2a. Example of a series of unidentified infrared-band spectra (HD44179) retrieved by the ISO Short Wavelength Spectrometer (SWS).
Figure 2b. Conceptual illustrations of lattice-vibration modes corresponding to each band. Circled numbers correspond to the numbers of bands in the chart (a).


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