2004ApJ...616.1167P

The origin of the so-called 21 µm feature that is especially prominent in the spectra of some carbon-rich protoplanetary nebulae (PPNs) is the matter of a lively debate. A large number of potential band carriers have been presented and discarded within the past decade. This paper gives an overview of the problems related to the hitherto proposed feature identifications, including the recently suggested candidate carrier silicon carbide. We also discuss the case for spectroscopically promising oxides. SiC is shown to produce a strong resonance band at 20-21 µm if coated by a layer of silicon dioxide. At low temperatures, core-mantle particles composed of SiC and amorphous SiO₂indeed have their strongest spectral signature at a position of 20.1 µm, which coincides with the position of the 21 µm emission band. The optical constants of another candidate carrier that has been relatively neglected so far–iron monoxide–are proven to permit a fairly accurate reproduction of the 21 µm feature profile as well, especially when low-temperature measurements of the infrared properties of FeO are taken into account. As candidate carrier of the 21 µm emission band, FeO has the advantage of being stable against further oxidation and reduction only in a narrow range of chemical and physical conditions, coinciding with the fact that this feature, too, is detected in a small group of objects only. However, it is unclear how FeO would form or survive, particularly in carbon-rich PPNs.