Origin of 21-Micron Emission Feature Is a Mystery

The proposed identification of the mysterious emission feature at 21 microns with titanium carbide nanoclusters (Physics Today, June 2000, page 21) is certainly a major news item in astrochemistry. However, no mention was made of the feature's initial discovery or of the 10 years of intensive research into this problem by astronomers and laboratory spectroscopists. Since 1989, when the 21-micron emission feature was discovered in four proto-planetary nebulae observed with the Infrared Astronomical Satellite,¹ many carriers, including large polycyclic aromatic hydrocarbon clusters, hydrogenated amorphous carbon grains, hydrogenated fullerenes, and nanodiamonds, have been proposed. These earlier suggestions were based in part on the great abundance of carbon observed in all of the nebulae that exhibited this emission feature.

The recent precise measurements of the feature's central wavelength (20.1 microns) and line profile based on observations by the Infrared Space Observatory² preceded the laboratory identification and made a definite identification possible.

Contrary to the impression given by the article that there are only two such objects, we have found 12 objects showing this emission feature, all belonging to a new class of celestial objects called proto-planetary nebulae. Why the 21-micron emission feature would be limited to such a short phase (a few thousand years) of stellar evolution is not understood.

Although the laboratory spectroscopy of titanium carbide clusters is a significant piece of work, it may not represent the final solution to the 21-micron feature mystery. A recent study has suggested that the feature can originate from out-of-plane bending modes of carbon rings with one carbon atom replaced by oxygen.³ Since the stretching and bending modes of aromatic hydrocarbons are commonly observed in proto-planetary nebulae, this suggestion is not unreasonable. Whatever the carrier of the 21-micron feature turns out to be, large-scale molecular synthesis leading to the formation of large organic molecules certainly can take place efficiently even in the low-density circumstellar environment. This may have implications for the question of the origin of life.