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A Critical Assessment of the PAH Hypothesis

Published online by Cambridge University Press:  23 September 2016

B. D. Donn ,
J. E. Allen  and
R. K. Khanna
B. D. Donn
Affiliation:
Code 690, Laboratory for Extraterrestrial Physics, NASA/Goddard Space Flight Center, Greenbelt, Maryland 20771
J. E. Allen
Affiliation:
Code 691, Astrochemistry Branch, NASA/Goddard Space Flight Center, Greenbelt, Maryland 20771
R. K. Khanna
Affiliation:
Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742

Abstract

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The proposal that polycyclic aromatic hydrocarbons (PAHs) are the source of the unidentified infrared bands has several serious deficiencies that have not been discussed or satisfactorily treated: (1) no collection of neutral PAHs has been found which matches the observed wavelengths; (2) ion or dehydrogenated molecules have been predicted to be the dominant species in some regions, but no infrared spectra of either species have been obtained; (3) the restriction to small grains is based on grain temperatures ∼ 1000-1500 K which in turn followed from the infrared continuum color temperature – there is now a question whether that is thermal emission; if it is, the source cannot be molecules; (4) recent observations of the 12/100 μm flux ratio as a function of stellar temperature do not conform to predictions of the PAH hypotheses; (5) the photon excitation mechanism for infrared emission by neutral molecules should produce strong visible-ultraviolet fluorescence which is not observed – ions may not do this, but their infrared spectra are not known. There does not seem to be a ready explanation for these problems with the PAH hypotheses. Until definitive experimental study and analysis yielding unambiguous results have been carried out for PAHs, hydrogenated amorphous carbon or other forms of carbonaceous material, it is premature to assume any type of grain is the source of the infrared bands.

Type
Section II: The Overidentified Infrared Emission Features
Information
Symposium - International Astronomical Union , Volume 135: Interstellar Dust , 1989 , pp. 181 - 189
Copyright
Copyright © Kluwer 1989 

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