Skip to main content

Thank you for visiting You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript.

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


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.

Your institute does not have access to this article

Relevant articles

Open Access articles citing this article.

Access options

Buy article

Get time limited or full article access on ReadCube.


All prices are NET prices.

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.


  1. Kwok, S. Organic Matter in the Universe (Wiley, 2011)

    Book  Google Scholar 

  2. Knacke, R. F. Carbonaceous compounds in interstellar dust. Nature 269, 132–134 (1977)

    CAS  ADS  Article  Google Scholar 

  3. Duley, W. W. & Williams, D. A. The infrared spectrum of interstellar dust: surface functional groups on carbon. Mon. Not. R. Astron. Soc. 196, 269–274 (1981)

    CAS  ADS  Article  Google Scholar 

  4. Allamandola, L. J., Tielens, A. G. G. M. & Barker, J. R. Interstellar polycyclic aromatic hydrocarbons: the infrared emission bands, the excitation/emission mechanism and the astrophysical implications. Astrophys. J. Suppl. Ser. 71, 733–775 (1989)

    CAS  ADS  Article  Google Scholar 

  5. Puget, J. L. & Léger, A. A new component of the interstellar matter: small grains and large aromatic molecules. Annu. Rev. Astron. Astrophys. 27, 161–198 (1989)

    CAS  ADS  Article  Google Scholar 

  6. Sellgren, K. The near-infrared continuum emission of visual reflection nebulae. Astrophys. J. 277, 623–633 (1984)

    CAS  ADS  Article  Google Scholar 

  7. Tielens, A. G. G. M. Interstellar polycyclic aromatic hydrocarbon molecules. Annu. Rev. Astron. Astrophys. 46, 289–337 (2008)

    CAS  ADS  Article  Google Scholar 

  8. Peeters, E. et al. The rich 6 to 9 μm spectrum of interstellar PAHs. Astron. Astrophys. 390, 1089–1113 (2002)

    CAS  ADS  Article  Google Scholar 

  9. Draine, B. T. & Li, A. Infrared emission from interstellar dust. IV. The silicate-graphite-PAH model in the post-Spitzer era. Astrophys. J. 657, 810–837 (2007)

    CAS  ADS  Article  Google Scholar 

  10. Uchida, K. I., Sellgren, K., Werner, M. W. & Houdashelt, M. L. Infrared Space Observatory mid-infrared spectra of reflection nebulae. Astrophys. J. 530, 817–833 (2000)

    CAS  ADS  Article  Google Scholar 

  11. Kwok, S., Volk, K. & Hrivnak, B. J. Chemical evolution of carbonaceous materials in the last stages of stellar evolution. Astron. Astrophys. 350, L35–L38 (1999)

    CAS  ADS  Google Scholar 

  12. Onaka, T. Interstellar dust: what do space observations tell us? Adv. Space Res. 25, 2167–2176 (2000)

    ADS  Article  Google Scholar 

  13. Clayton, G. C. et al. The role of polycyclic aromatic hydrocarbons in ultraviolet extinction. I. Probing small molecular polycyclic aromatic hydrocarbons. Astrophys. J. 592, 947–952 (2003)

    CAS  ADS  Article  Google Scholar 

  14. Pendleton, Y. J. & Allamandola, L. J. The organic refractory material in the diffuse interstellar medium: mid-infrared spectroscopic constraints. Astrophys. J. Suppl. Ser. 138, 75–98 (2002)

    CAS  ADS  Article  Google Scholar 

  15. Pilleri, P. et al. Search for corannulene (C20H10) in the Red Rectangle. Mon. Not. R. Astron. Soc. 397, 1053–1060 (2009)

    CAS  ADS  Article  Google Scholar 

  16. Guillois, O., Nenner, I., Papoular, R. & Reynaud, C. Coal models for the infrared emission spectra of proto-planetary nebulae. Astrophys. J. 464, 810–817 (1996)

    CAS  ADS  Article  Google Scholar 

  17. Kahanpää, J., Mattila, K., Lehtinen, K., Leinert, C. & Lemke, D. Unidentified infrared bands in the interstellar medium across the Galaxy. Astron. Astrophys. 405, 999–1012 (2003)

    ADS  Article  Google Scholar 

  18. Sakata, A., Wada, S., Onaka, T. & Tokunaga, A. T. Infrared spectrum of quenched carbonaceous composite (QCC). II. A new identification of the 7.7 and 8.6 micron unidentified infrared emission bands. Astrophys. J. 320, L63–L67 (1987)

    CAS  ADS  Article  Google Scholar 

  19. Duley, W. W. & Hu, A. Polyynes and interstellar carbon nanoparticles. Astrophys. J. 698, 808–811 (2009)

    CAS  ADS  Article  Google Scholar 

  20. Jäger, C., Huisken, F., Mutschke, H., Jansa, I. L. & Henning, T. H. Formation of polycyclic aromatic hydrocarbons and carbonaceous solids in gas-phase condensation experiments. Astrophys. J. 696, 706–712 (2009)

    ADS  Article  Google Scholar 

  21. Duley, W. W. & Williams, D. A. Excitation of the aromatic infrared emission bands: chemical energy in hydrogenated amorphous carbon particles? Astrophys. J. 737, L44 (2011)

    ADS  Article  Google Scholar 

  22. Papoular, R., Conrad, J., Giuliano, M., Kister, J. & Mille, G. A coal model for the carriers of the unidentified IR bands. Astron. Astrophys. 217, 204–208 (1989)

    CAS  ADS  Google Scholar 

  23. Papoular, R. The use of kerogen data in understanding the properties and evolution of interstellar carbonaceous dust. Astron. Astrophys. 378, 597–607 (2001)

    CAS  ADS  Article  Google Scholar 

  24. Kwok, S. The synthesis of organic and inorganic compounds in evolved stars. Nature 430, 985–991 (2004)

    CAS  ADS  Article  Google Scholar 

  25. Evans, A. et al. Infrared spectroscopy of nova Cassiopeiae 1993. IV. A closer look at the dust. Mon. Not. R. Astron. Soc. 360, 1483–1492 (2005)

    CAS  ADS  Article  Google Scholar 

  26. Ney, E. P. & Hatfield, B. F. The isothermal dust condensation of Nova Vulpeculae 1976. Astrophys. J. 219, L111–L115 (1978)

    ADS  Article  Google Scholar 

  27. Kwok, S., Volk, K. & Bernath, P. On the origin of infrared plateau features in proto-planetary nebulae. Astrophys. J. 554, L87–L90 (2001)

    CAS  ADS  Article  Google Scholar 

  28. Derenne, S. & Robert, F. Model of molecular structure of the insoluble organic matter isolated from Murchison meteorite. Meteorit. Planet. Sci. 45, 1461–1475 (2010)

    CAS  ADS  Article  Google Scholar 

  29. Cody, G. D. et al. Establishing a molecular relationship between chrondritic and cometary organic solids. Proc. Natl Acad. Sci. . USA advance online publication, 〈〉 (4 April 2011)

  30. Sellgren, K., Uchida, K. I. & Werner, M. W. The 15–20 μm Spitzer spectra of interstellar emission features in NGC 7023. Astrophys. J. 659, 1338–1351 (2007)

    CAS  ADS  Article  Google Scholar 

Download references


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).

Author information

Authors and Affiliations



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

Corresponding author

Correspondence to Sun Kwok.

Ethics declarations

Competing interests

The authors declare no competing financial interests.

PowerPoint slides

Rights and permissions

Reprints and Permissions

About this article

Cite this article

Kwok, S., Zhang, Y. Mixed aromatic–aliphatic organic nanoparticles as carriers of unidentified infrared emission features. Nature 479, 80–83 (2011).

Download citation

  • Received:

  • Accepted:

  • Published:

  • Issue Date:

  • DOI:

Further reading


By submitting a comment you agree to abide by our Terms and Community Guidelines. If you find something abusive or that does not comply with our terms or guidelines please flag it as inappropriate.


Quick links

Nature Briefing

Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily.

Get the most important science stories of the day, free in your inbox. Sign up for Nature Briefing