2011ApJ...738..121C

Among its great findings, the Infrared Astronomical Satellite mission showed the existence of an unidentified mid-IR feature around 21 µm. Since its discovery, this feature has been detected in all C-rich protoplanetary nebulae (PPNe) of intermediate spectral type (F-G) and–weakly–in a few PNe and asymptotic giant branch (AGB) stars, but the nature of its carriers remains unknown. In this paper, we show the detection of this feature in the spectra of three new stars transiting from the AGB to the PN stage obtained with the Spitzer Space Telescope. Following a recent suggestion, we try to model the spectral energy distributions of our targets with amorphous carbon and FeO, which might be responsible for the unidentified feature. The fit thus obtained is not completely satisfactory, since the shape of the feature is not well matched. In an attempt to relate the unidentified feature to other dust features, we retrieved mid-IR spectra of all 21 µm sources currently known from Infrared Space Observatory and Spitzer online archives and noticed a correlation between the flux emitted in the 21 µm feature and that emitted at 7 and 11 µm (polycyclic aromatic hydrocarbon bands and hydrogenated amorphous carbon broad emission). Such a correlation may point to a common nature of the carriers.