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The mysterious "21 micrometer" emission feature seen almost exclusively inthe short-lived protoplanetary nebula (PPN) phase of stellar evolution remainsunidentified since its discovery two decades ago. This feature is alwaysaccompanied by the equally mysterious, unidentified "30 micrometer" feature andthe so-called "unidentified infrared" (UIR) features at 3.3, 6.2, 7.7, 8.6, and11.3 micrometer which are generally attributed to polycyclic aromatichydrocarbon (PAH) molecules. The 30 micrometer feature is commonly observed inall stages of stellar evolution from the asymptotic giant branch (AGB) throughPPN to the planetary nebula phase.  We explore the interrelations among the mysterious 21 micrometer, 30micrometer, and UIR features in the Galactic and Magellanic Cloud of the 21micrometer sources. We derive the fluxes emitted in the observed UIR, 21micrometer, and 30 micrometer features from published ISO or Spitzer/IRSspectra.  We find that none of these spectral features correlate with each other. Thisargues against a common carrier (e.g., thiourea) for both the 21 micrometerfeature and the 30 micrometer feature (otherwise these two features shouldcorrelate). This also does not support large PAH clusters as a possible carrierfor the 21 micrometer feature.

astrophysical journal/the astrophysical journal

A Tale of Three Mysterious Spectral Features in Carbon-Rich Evolved  Stars: The 21 Micrometer, 30 Micrometer, and "Unidentified Infrared" Emission  Features