New types of dust identified by infrared satellites’ observations
A majority of lattice vibrations of solid substances reside in the infrared region. Recent satellite observations discovered a number of previously unknown dust bands, and identified the existence of new types of dust. Until now, it was thought that stones, which are not crystalline like glass, were a major component of interstellar dust. With ISO’s spectroscopic observations of old and young stars, however, it became evident that crystalline stones are present as well. Because it is linked to the origin of the solar system, the hot topic now is the question of how crystallization progresses through its stages of evolution and by what processes.
Bands from crystalline stones are seen around stars but not in the thermal radiation from general interstellar space. Therefore, they are still a minor part of the dust. New bands are observed in the thermal-radiation component from interstellar space, like the PAH bands, as well. For example, Fig. 2 shows a broad emission band (solid line) in 22 24 microns, which was detected by ISO only in regions where massive stars are being born. Observations by the Spitzer Space Telescope confirmed the existence of this band (dashed line in Fig. 2). The observation also indicated that the band exists only in a relatively limited ionized region. If the band is related to the birth of massive stars, there is a possibility that the dust is produced by supernova explosions or is processed by the explosions. The Spitzer Space Telescope also detected a similar band in galaxies where relatively new stars were born. If it can be proved that the band is closely related to star-formation phenomenon, it would give us an effective tool to hunt for just-born galaxies. Also, this is the first dust to have its origins clarified, unlike stone and coal whose formation origins still remain unclear. Although we were able to specify the origin, their real nature is still unclear. To begin with, since there is no clue to characterize it except for the broader band, it is very difficult to determine its composition. A variety of hypotheses have been proposed, such as iron oxide or just-made stones, but as yet, no decisive conclusion has been presented. After additional observations of many celestial objects, its real identity is expected to be revealed.
Fig. 3 shows the most suspicious dust bands discovered by ISO. One is around 65 microns, which is also an emission band observed in star-formation regions. It is known that a crystalline stone called diopside containing calcium has a similar band (inset figure in Fig. 3). If this identification is correct, it would be the first crystalline dust found in general interstellar space.
Looking at the spectrum in more detail, a somewhat broad, weak band is faintly seen around 100 microns. For stars where a similar band has been discovered, carbonate dust is proposed as a candidate. The band seen in the star-formation region, however, seems to have a trail extending on the longer wavelength side. We speculated that this might be a band emission from fine particles of carbon onion, a roll of graphitic sheets. Of course, we cannot get decisive evidence from just a broad, weak band. Since the carbon onion has been proposed to explain the absorption band in the ultraviolet, however, it may not be such a wild guess.