Abstract
The 21-μm and 30-μm bands are the strongest dust emission features detected in evolved low- and intermediate-mass C-rich stars (i.e. asymptotic giant branch (AGB) stars, proto-planetary nebulae (PPNe), and planetary nebulae (PNe)). While the 21-μm feature is rare and exists only in the transient PPN phase, the 30-μm feature is more common and seen in the entire late stage of stellar evolution, from AGB to PPN and PN phases, as well as in the low-metallicity galaxies: the Large Magellanic Cloud (LMC) and the Small Magellanic Cloud (SMC). The carriers of these features remain unidentified. All the 21-μm sources also emit in the 30-μm band, suggesting that their carriers may be somewhat related.
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