The following document lists the file abstract/SKWOK_21MICRON.abs
from catalogue VI/111.
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Searches of the IRAS LRS database have revealed a small number of sources with an unidentified emission feature at 21 microns (Kwok, Volk, & Hrivnak 1989, ApJ, 387, L89). Ground-based follow-up observations have found that this feature occurs only in F and G super- giants with large infrared excesses, and almost all 21 micron sources have been identified as stars in transition from the asymptotic giant branch to the planetary nebulae phase. Optical spectroscopy of the 21 micron sources have shown all of them to be carbon rich, and they often display strong 3.3, 3.4, and 11.3 micron UIR emission features. In order to identify the origin of the 21 micron feature, it is necessary to obtain higher spectral resolution data to determine the fine structures of the feature. We propose to use the SWS in the grating mode to observe 5 of the brightest 21 micron sources which have been discovered and observed by us from the ground. We also request observations of 6 other candidate 21 micron feature sources identified by UKIRT ground-based spectroscopy near 20 microns.