2015ApJ...802...39M

The mysterious ''21 µm'' emission feature seen almost exclusively in the short-lived protoplanetary nebula (PPN) phase of stellar evolution remains unidentified since its discovery two decades ago. This feature is always accompanied by the equally mysterious, unidentified ''30 µm'' feature and the so-called ''unidentified infrared'' (UIR) features at 3.3, 6.2, 7.7, 8.6, and 11.3 µm which are generally attributed to polycyclic aromatic hydrocarbon (PAH) molecules. The 30 µm feature is commonly observed in all stages of stellar evolution from the asymptotic giant branch through PPN to the planetary nebula phase. We explore the interrelations among the mysterious 21, 30 µm, and UIR features of the 21 µm sources. We derive the fluxes emitted in the observed UIR, 21, and 30 µm features from published Infrared Space Observatory or Spitzer/IRS spectra. We find that none of these spectral features correlate with each other. This argues against a common carrier (e.g., thiourea) for both the 21 µm feature and the 30 µm feature. This also does not support large PAH clusters as a possible carrier for the 21 µm feature.