Possible assignment of the 11.3-μm UIR feature to emission from carbonaceous microparticles with internal hydrogens

Abstract

Summary The carriers of the UIR bands seen in many galactic and extragalactic objects have previously been assigned to emission from polycyclic aromatic hydrocarbons (PAHs). However, the match between laboratory spectra of typical PAHs with the interstellar features in the structurally sensitive 11-15-μm (909-667-cm-1) region remains unsatisfactory. In particular, it has been difficult to explain, considering that a PAH mixture is undoubtedly involved, why only one strong band is seen at 11.3 μm (885 cm-1) and why this has the same position in all sources. We discuss here the possibility that a hitherto unrecognized class of molecule containing a novel type of hydrogen functional group, the internal hydrogen, may provide important new clues to the origin of the 11.3-μm (885 cm-1) feature. This type of grouping is typified by the molecule kekulene which contains six such internal hydrogens and exhibits bands in its IR absorption spectrum which coincide with the interstellar feature. This structural unit is likely to be common in soot-like microparticles which simulations suggest are present in the cool carbon-rich outflows of red giant stars and hence should contribute to the 11.3-μm (885-cm-1) emission.