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Mixed aromatic–aliphatic organic nanoparticles as carriers of unidentified infrared emission features

Nature volume 479, pages 80–83 (2011) Cite this article

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Abstract

Unidentified infrared emission bands at wavelengths of 3–20 micrometres are widely observed in a range of environments in our Galaxy and in others1. Some features have been identified as the stretching and bending modes of aromatic compounds2,3, and are commonly attributed to polycyclic aromatic hydrocarbon molecules4,5. The central argument supporting this attribution is that single-photon excitation of the molecule can account for the unidentified infrared emission features observed in ‘cirrus’ clouds in the diffuse interstellar medium6. Of the more than 160 molecules identified in the circumstellar and interstellar environments, however, not one is a polycyclic aromatic hydrocarbon molecule. The detections of discrete and broad aliphatic spectral features suggest that the carrier of the unidentified infrared emission features cannot be a pure aromatic compound. Here we report an analysis of archival spectroscopic observations and demonstrate that the data are most consistent with the carriers being amorphous organic solids with a mixed aromatic–aliphatic structure. This structure is similar to that of the organic materials found in meteorites, as would be expected if the Solar System had inherited these organic materials from interstellar sources.

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Figure 1: Mixed aromatic and aliphatic features in the infrared spectra of circumstellar and interstellar nebulae.
Figure 2: Proposed structure of the carrier of UIE features.
Figure 3: Emergence of complex organics after nova outburst.

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Acknowledgements

We thank A. Tang for technical assistance in the preparation of this manuscript. This work was supported by a grant to S.K. from the Research Grants Council of the Hong Kong Special Administrative Region, China (project no. HKU 7027/11P).

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Authors and Affiliations

  1. Department of Physics, Faculty of Science, The University of Hong Kong, Pokfulam Road, Hong Kong, China,

    Sun Kwok & Yong Zhang

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  1. Sun Kwok
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  2. Yong Zhang
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Contributions

S.K. designed the research and wrote the paper. Y.Z. performed data analysis and model fitting.

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Correspondence to Sun Kwok.

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Kwok, S., Zhang, Y. Mixed aromatic–aliphatic organic nanoparticles as carriers of unidentified infrared emission features. Nature 479, 80–83 (2011). https://doi.org/10.1038/nature10542

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Comments

Commenting on this article is now closed.

  1. David Smith

    I enjoyed reading this paper from Kwok and Zhang, and am impressed by the use of infrared emission to detect aliphatic species in a range of deep space environments. This observation adds significantly to the polycyclic aromatic hydrocarbon hypothesis as outlined by the authors, and indicates that aliphatic species may play an important role in chemical evolution.

    However, I must urge significant caution about the use of Figure 2. The authors suggest this is a 'proposed structure'. This would indicate there is evidence for this actual molecule, or at least this type of molecule and/or collection of functional groups. However, this is not supported by the data.

    The infrared data indicate the presence of both aliphatic and aromatic components. However, the presence of heteroatoms within the structure is not evidenced (it is postulated based on the presence of these elements in cosmic gases). Although the incorporation of these elements into the structures is completely plausible, there is no experimental evidence, as yet for their incorporation &#8211 it is only suggested they may be detected at higher resolution. Furthermore, if these elements are part of the actual organic structures, they should not be described as 'impurities'.

    The actual detailed specifics of the structure are certainly not supported by the results in the paper. No evidence is provided here for any of the heterocyclic structures, or the presence of aldehydes, nitriles, alcohols, esters, carboxylic acids, ethers etc. Of course some of these may well be present in the organic molecules found in space, but a chemical structure of this type should only really be presented in the face of direct evidence for each type of functional group. Finally, I would note that the structure contains two incorrect singly bonded oxygen atoms (which should carry either a negative charge, or a hydrogen atom).

    In summary, it is my view that this paper contains significant data, but that Figure 2 is a misrepresentation of the results obtained.

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