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

Abstract

The mysterious "21 micrometer" 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 micrometer" feature and the so-called "unidentified infrared" (UIR) features at 3.3, 6.2, 7.7, 8.6, and 11.3 micrometer which are generally attributed to polycyclic aromatic hydrocarbon (PAH) molecules. The 30 micrometer feature is commonly observed in all stages of stellar evolution from the asymptotic giant branch (AGB) through PPN to the planetary nebula phase. We explore the interrelations among the mysterious 21 micrometer, 30 micrometer, and UIR features in the Galactic and Magellanic Cloud of the 21 micrometer sources. We derive the fluxes emitted in the observed UIR, 21 micrometer, and 30 micrometer features from published ISO 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 micrometer feature and the 30 micrometer feature (otherwise these two features should correlate). This also does not support large PAH clusters as a possible carrier for the 21 micrometer feature.

18 pages, 8 figures, 3 tables; published in The Astrophysical Journal, 2015, vol. 802, pp. 39

References in corpus (5)

• Infrared Emission from Interstellar Dust. IV. The Silicate-Graphite-PAH Model in the Post-Spitzer Era
• Preplanetary Nebulae: An HST Imaging Survey and a New Morphological Classification System
• Physical Properties of Fullerene-containing Galactic Planetary Nebulae
• On Iron Monoxide Nanoparticles as a Carrier of the Mysterious 21 Micrometer Emission Feature in Post-Asymptotic Giant Branch Stars
• A Tale of Three Galaxies: Anomalous Dust Properties in IRASF10398+1455, IRASF21013-0739 and SDSSJ0808+3948

Cited by in corpus (5)

• Hidden IR structures in NGC 40: signpost of an ancient born-again event
• On the Relation between the Mysterious 21 Micrometer Emission Feature of Post-Asymptotic Giant Branch Stars and Their Mass Loss Rates
• Molecular gas in 21 and 30 micron sources: the 2\,mm and 1.3\,mm spectra of IRAS\,21318+5631 and 22272+5435
• Warm CO in evolved stars from the THROES catalogue. II. Herschel/PACS spectroscopy of C-rich envelopes
• On the formation of molecules and solid-state compounds from the AGB to the PN phases