2016ApJ...825...68M

Over two decades ago, a prominent, mysterious emission band peaking at ∼20.1 µm was serendipitously detected in four preplanetary nebulae (PPNe; also known as "protoplanetary nebulae"). To date, this spectral feature, designated as the "21 µm" feature, has been seen in 27 carbon-rich PPNe in the Milky Way and the Magellanic Clouds. The nature of its carriers remains unknown although many candidate materials have been proposed. The 21 µm sources also exhibit an equally mysterious, unidentified emission feature peaking at 30 µm. While the 21 µm feature is exclusively seen in PPNe, a short-lived evolutionary stage between the end of the asymptotic giant branch (AGB) and planetary nebula (PN) phases, the 30 µm feature is more commonly observed in all stages of stellar evolution from the AGB through PPN to PN phases. We derive the stellar mass-loss rates ( {dot}M ) of these sources from their infrared (IR) emission, using the "2-DUST" radiative transfer code for axisymmetric dusty systems which allows one to distinguish the mass-loss 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 µm features. We find that both features tend to correlate with {dot}M_AGB , suggesting that their carriers are probably formed in the AGB phase. The nondetection of the 21 µm feature in AGB stars suggests that, unlike the 30 µm feature, the excitation of the carriers of the 21 µm feature may require ultraviolet photons which are available in PPNe but not in AGB stars.