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Over two decades ago, a prominent, mysterious emission band peaking at ~20.1micrometer was serendipitously detected in four preplanetary nebulae (PPNe;also known as "protoplanetary nebulae"). So far, this spectral feature,designated as the "21 micrometer" feature, has been seen in 18 carbon-richPPNe. The nature of the carriers of this feature remains unknown although manycandidate materials have been proposed. The 21 micrometer sources also exhibitan equally mysterious, unidentified emission feature peaking at 30 micrometer.While the 21 micrometer feature is exclusively seen in PPNe, a short-livedevolutionary stage between the end of the asymptotic giant branch (AGB) andplanetary nebula (PN) phases, the 30 micrometer feature is commonly observed inall stages of stellar evolution from the AGB through PPN to PNe phases. Wederive the stellar mass loss rates (M_{loss}) of these 21 micrometer sourcesfrom their dust infrared (IR) emission, using the "2-DUST" radiative transfercode for axisymmetric dusty systems which allows one to distinguish the massloss rates of the AGB phase (\dot{M_{AGB}}) from that of the superwind(\dot_{M_{SW}}) phase. We examine the correlation between \dot{M_{AGB}} or\dot_{M_{SW}} and the fluxes emitted from the 21 and 30 micrometer features. Wefind that both features tend to correlate with \dot{M_{AGB}}, suggesting thattheir carriers are probably formed in the AGB phase. The nondetection of the 21micrometer feature in AGB stars suggests that, unlike the 30 micrometerfeature, the excitation of the carriers of the 21 micrometer feature mayrequire ultraviolet photons which are available in PPNe but not in AGB stars.

On the Relation between the Mysterious 21 Micrometer Emission Feature of Post-Asymptotic Giant Branch Stars and Their Mass Loss Rates