8/3/2019 Nature 10542

1/4

LETTERdoi:10.1038/nature10542

Mixed aromaticaliphatic organic nanoparticles ascarriers of unidentified infrared emission featuresSun Kwok1 & Yong Zhang1

Unidentified infrared emission bands at wavelengths of 320micrometres are widely observed in a range of environments inour Galaxy and in others1. Some features have been identified asthe stretching and bending modes of aromatic compounds2,3, andare commonly attributed to polycyclic aromatic hydrocarbonmolecules4,5. The central argument supporting this attribution isthat single-photon excitation of the molecule can account for theunidentified infrared emission features observed in cirrus cloudsin the diffuse interstellar medium6. Of the more than 160 mol-ecules identified in the circumstellar and interstellar environ-

ments, however, not one is a polycyclic aromatic hydrocarbonmolecule. The detections of discrete and broad aliphatic spectralfeatures suggest that the carrier of the unidentified infrared emis-sion features cannotbe a pure aromatic compound. Here we reportan analysis of archival spectroscopic observations and demonstratethat the data are most consistent with the carriers being amorph-ous organic solids with a mixed aromaticaliphatic structure. Thisstructure is similar to that of the organic materials found inmeteorites, as would be expected if the Solar System had inheritedthese organic materials from interstellar sources.

For the past 20 years, polycyclic aromatic hydrocarbon (PAH) mol-ecules have commonly been considered the carriers of unidentifiedinfrared emission (UIE) features. This hypothesis assumes that theUIE features are the result of infrared fluorescence from small (,50-carbon-atom)gas-phase PAH molecules beingpumpedby far-ultravioletphotons7. In spite of its popularity, the PAH hypothesis does not provide

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

2.0

1.5

1.0

0.5

0.0

0.5

0.0

0.1

0.2

0.3

0.4

0.5

0.0

0.1

0.2

0.3

0.4

0.8

0.6

0.7

2 4 6 8 10 12 14 16 18 20

2 4 6 8 10 12 14 16 18 20

4 6 8 10 12 14 16 18 20 22 24

3.3

6.2

7.6 7.8

11.3

12.1

12.716.4

[MgV]

[Si IV]

[NeV]

[S III]

[Ne III]

8-m plateau

12-m plateau

NGC 7027

6.2

6.97.7

11.312.2

13.4 15.8

8-m plateau

12-m plateau

IRAS 22272+5435

F

(1010Wm

2)

F

(

1010Wm

2)

F

(

1010Wm

2)

Wavelength (m)

20.1

a

b

c

8-m plateau 12-m plateau

6.27.6

7.8

11.2

16.4

8.6 13.5

[S III]

[Ne III]

[Ne II]Orion bar

17-m plateau

Figure 1 | Mixed aromatic and aliphatic features in the infrared spectra ofcircumstellar and interstellar nebulae. ac, Spectral decompositions of theUIEfeatures of theplanetary nebulaNGC 7027(a), the proto-planetary nebulaIRAS2227215435(b) andthe Orionbar photodissociation region(c), showinga mix of aromatic, aliphatic and continuum features. The observed flux atwavelength l, Fl, is proportional to theemissionintensity at that wavelength. Aseries of discrete features (black lines) and plateau features (orange lines; inc, the 17-mm plateau represents the 1520-mm range) superposed on acontinuum (blue line) have been fitted to the observed data. The UIE andplateau features (wavelengths in micrometres), as well as some of the atomiclines, are marked. The observed spectra are shown as solid red lines and thefittedspectra are shown as dottedblacklines. Theorigin of the 20.1-mm feature

in the IRAS 2227215435 spectrum is currently unidentified. The spectral datafor NGC 7027, IRAS 2227215435 and the Orion bar are retrieved from theInfrared Space Observatory archive. For the spectral decomposition, we usedthe IDL package PAHFIT originallydeveloped to fit the Spitzer SpaceTelescopeInfrared Spectrograph spectra of nearby galaxies. The model spectra take intoaccount the contributions from the stellar continuum, the thermal dustcontinuum, H2 emission, atomic emission lines, the UIE features (botharomatic and aliphatic)and the plateau emission features.The optimal fitting tothe observed spectra is achieved through the LevenbergMarquardt least-squares algorithm. A modified blackbody model for the emission intensity atwavelength l, Il / l

2aBl(T), where Bl(T) is the blackbody function with atemperatureT, is used to fitthe continuum. Thearomatic, aliphaticand plateaufeatures are fitted with assumed Drude profiles Il / c

2[(l/l02 l0/l)21 c2]21,

where l0 is the central wavelength and c is the fractional full-width at half-maximum of each feature.

0 0 M O N T H 2 0 1 1 | V O L 0 0 0 | N A T U R E | 1

Macmillan Publishers Limited. All rights reserved2011

http://www.nature.com/doifinder/10.1038/nature10542http://www.nature.com/doifinder/10.1038/nature10542

8/3/2019 Nature 10542

2/4

a good explanation for the observed spectral behaviour. PAH moleculesare fused ring molecules made up of carbon and hydrogen, and their

vibrational bands aresharp and the peak wavelengths well defined. To fitthe broad profiles of the UIE features seen in astronomical spectra, it isnecessary to use a complex mixture of PAHs of different sizes, structuresandchargestates, andto utilizeempirical featureprofiles8,9. BecausePAHmolecules require ultraviolet photonsto excite them, theycannot explainthe presence of UIE features in reflection nebulae10 and proto-planetary

nebulae11

where the central stars are cool and there is no ultravioletbackground radiation. To account for these facts, the PAH model hasto be revised to include large clusters and other ionization states.

The central argument for the PAH hypothesis is that single-photonexcitation of PAH molecules can account for the 12-mm excess emis-sion observed in cirrusclouds in the diffuse interstellarmedium by theInfrared Astronomical Satellite (IRAS). However, the UIE-band fluxratios in the diffuse H II regions in the Carina nebula are nearly con-stant over a range of three orders of magnitude in background radi-ation12. The shapes and peak wavelengths of the UIE features areindependent of the temperature of the central stars providing theexcitation10. Furthermore, PAH molecules have strong and narrowabsorption features in the ultraviolet, but these are not observed ininterstellar extinctioncurves13. However, the3.4-mm aliphaticcarbon

hydrogen stretching mode is commonly observed in absorption in thediffuse interstellar medium14. Although their rotational and vibra-tional frequencies are well known, not a single PAH molecule hasyet been identified in space15.

Other arguments have been made to support the PAH hypo-thesis: the asymmetric profiles of the UIE features can be explainedby anharmonicity associated with molecular emission, and theobserved feature-to-continuum ratiois high and therefore implies thatthe carrier is a molecule7. Laboratory spectra of mixed aromatic andaliphatic solid materials have asymmetric profiles16, which can more

naturally explain the observations. Although the observed feature-to-continuumratio is high in the diffuse interstellar medium, it is nothighin the spectra of planetary and proto-planetary nebulae. Even in thediffuse interstellar medium, the strength of the UIE features arestrongly correlated with the dust continuum, suggesting a possiblephysical relationship between the two components17.

The basic premise of the PAH hypothesis does not concern thechemical composition of the carrier so much as its size. As long as

the carrier is a nanoparticle that can undergo transient heating, it willsatisfy the excitation requirement. Laboratory experiments haveyielded carbon nanoparticles with structures of sp2 rings connectedby networks of aliphatic chains18,19, as well as fullerene fragmentslinked by aliphatic groups20. These nanoparticles are likely to be con-stituents in circumstellar and interstellar environments. Furthermore,it has been proposed that the possible sudden release of chemicalenergy as a source of transient heating of small grains will allow muchlarger particles to radiate in the near-infrared, further weakening thePAH hypothesis21.

An alternative explanation to the UIE bands is that they are emittedby complex organic solids with disorganized structures. These solidsintrinsically have broad emission profiles, and the features often sit oneven broader emission plateaux several micrometres in width. It has

been argued for some time that theobserved spectral properties of UIEbands resemble those of coal and kerogen22,23. Coal and kerogen areamorphous organic solids with a mixed sp2sp3 composition withrandomly oriented aromatic ring unitslinked by long, aliphatic chains.Their mixed sp2sp3 chemistry gives rise to the discrete aromatic andaliphatic emission features and the broad plateau features16.

To provide a quantitative comparison between the two models, wehave performed spectral decomposition of several sources with strongUIEfeatures.Figure1 showsa fitto theinfrared spectra ofthe planetarynebula NGC 7027, the proto-planetary nebula IRAS 2227215435 and

Table 1 | Strengths of the UIE discrete and plateau features

Discrete features* (%)

Aromatic Aliphatic Unknown

l (mm) 3.3 6.2 7.7 8.6 11.3 3.4 6.9 15.8 16.4 18.9NGC 7027 0.32 1.2 2.8 0.58 3.1 0.07 0.11 0.31 0.84 0.0IRAS 2227215435{ 0.08 0.05 0.30 0.11 3.76 0.15 0.43 0.94 0.41 Orion bar 0.67 3.8 7.0 2.2 2.6 0.13 0.37 0.24 2.0 0.0V2361 Cygni 0.27{ 0.871 0.60I 0.03 0.25 V2362 Cygni 5.2 2.4 0.8 3.2

Plateau features (%)

Aromatic Aliphatic Unknown

l (mm) 8 12 17NGC 7027 18.8 17.2 0.33IRAS 2227215435 12.5 18.6 Orion bar 9.3 15.1 0.78V2361 Cygni 11.1 1.3 V2362 Cygni 17.1 1.2 1.1

Continuum Total flux (3-20 mm) (W m-2)

P ercen tag e o f tota l flux Temper atu re (K) a

NGC 7027 50.7 100 2 8.9 (211)IRAS 2227215435 40.8 100 2 2.5 (211)Orion bar 47.3 70 2 2.4 (211)V2361 Cygni 84.8 350 0.2 1.8 (213)V2362 Cygni 66.8 365 0.5 2.1 (213)

TheUIEphenomenonis complex.In additiontothecommonlyobserved3.3-,6.2-,7.7-,8.6- and11.3-mmaromaticfeatures,therearealsoaliphaticfeaturesat 3.4and6.9 mm,arisingrespectivelyfromsymmetricand

asymmetriccarbonhydrogenstretchingandbendingmodesof methylandmethylenegroups attachedtoaromaticrings.Featuresat 15.8,16.4,17.4(notshown),17.8 (notshown),and18.9mmhavebeenfoundin

proto-planetary nebulae11, reflectionnebulae30 andgalaxies.In additionto thediscretefeatures,broademission featuresup toseveralmicrometresin width arealso seen. The8- and12-mm plateaufeatures anda

broad feature covering the 1520-mm range(represented by 17mm inthe table) havebeen detectedin young stellar objects, compact H II regions andplanetary nebulae. The8- and12-mm plateaux are broad

emissionfeatures (full-widthat half-maximum,24 mm) andcanbe identified as collective in-planeand,respectively, out-of-planebendingmodesof a mixtureof aliphatic sidegroupsattachedto aromaticrings27.

The1520-mm plateau feature is alsofoundto be strong in someproto-planetarynebulae. This table summarizes therelativecontributions of these components of the spectra shown in Figs 1 and 3.

*The total fluxes and percentages refer to the values emitted in the 320 mm range for NGC 7027 and Orion bar, 520 mm for V2362 Cygni and V2361 Cygni, and 325 mm for IRAS 2227215435.

{ForIRAS 2227215435,thereare additionalfeaturesthatcontributeto thefluxin the325-mmrange.Featuresat 12.2,13.4and 20.1mm contribute3.83, 1.53and 2.56%, respectively.Some ofthe contributions

also come from the broad 26-mm feature.

{This entry refers to the spectral feature at 5.3 mm.

1This entry refers to the spectral feature at 6.3 mm.

IThis entry refers to the spectral feature at 7.2 mm.

RESEARCH LETTER

2 | N A T U R E | V O L 0 0 0 | 0 0 M O N T H 2 0 1 1

Macmillan Publishers Limited. All rights reserved2011

8/3/2019 Nature 10542

3/4

the Orion bar, a photodissociation region in the Orion nebula, using aset of discrete UIE features, broad plateau features, and the underlyingdust continuum. The breakdown of contributions to the total fluxesfrom various components is summarized in Table 1. The strongestcomponent is the continuum, which contributes approximately halfof the total fluxes emitted in the 320-mm region. The next strongestare the plateau features, which account for 36, 31 and 25% of the totalfluxesfrom NGC7027, IRAS 2227215435 and theOrion bar, respect-

ively, compared with the totals of 8, 4 and 16% from the aromaticfeatures. The aliphatic branches probably constitute a significant frac-tion of the material in each of the three sources.

The above fitting results show that the carrier of the UIE featuresincludes a mixture of aromatic and aliphatic components, and is not apure or predominantly aromatic compound. Because the carrier isformed from a mixture of cosmic gases, it is likely that the compoundwill include other abundant elements such as oxygen, nitrogen, sul-phur and so on, in addition to carbon and hydrogen. These impuritiesmayalso have spectralsignatures thatcan be identified by observationsat higher spectral resolution. A sketch of the proposed chemical struc-ture is shown in Fig. 2.

The best way to study the origin of the UIE features is to observethem when they are formed. From observations of objects in the latestages of stellar evolution,we know that theUIE features develop in thecircumstellar environment within a few hundred years after the ter-mination of the asymptotic giant branch24. Spectroscopic observationsof novae have shown that the 3.3- and 3.4-mm features appear soonafter dust condensation25. Theoretically, it is difficult to understandhow complex organics can form under such low-density conditions,but novae are observed to change from a pure gas spectrum to a dust-dominated spectrum over the course of days26. In Fig. 3, we show a fitto Spitzer spectra of the novae V2362 Cygni and V2361 Cygni. It isexpected that a mixture of miscellaneous aliphatic branches will attachto the newly formed ring clusters. The prominence of the plateaufeatures reflects this early stage of organic dust condensation27.

We note that the dominant organic content in carbonaceous chon-drites is a kerogen-like macromolecular solid referred to as insolubleorganic matter. Recent laboratory analysis of the insoluble organic

matter in theMurchison meteorite hassuggestedthat it hasa chemicalstructure very similar to that which we propose here28,29. The presenceof insoluble organic matter in meteorites is evidence that complex

organic solids form in naturewith no difficulty. The fact that insoluble

organic matter and circumstellar dusthavesimilar chemical structuresoffers the possibility that Solar System organics may have a stellarconnection.

Received 1 April; accepted 30 August 2011.

Published online 26 October 2011.

1. Kwok, S. Organic Matter in the Universe (Wiley, 2011).2. Knacke,R. F. Carbonaceouscompounds in interstellardust. Nature 269, 132134

(1977).3. Duley, W. W. & Williams, D. A. The infrared spectrum of interstellar dust: surface

functional groups on carbon. Mon. Not. R. Astron. Soc. 196, 269274 (1981).4. Allamandola, L. J., Tielens, A. G. G. M. & Barker, J. R. Interstellar polycyclic

aromatic hydrocarbons: the infrared emission bands, the excitation/emissionmechanism and the astrophysical implications. Astrophys. J. Suppl. Ser. 71,733775 (1989).

5. Puget,J.L.&Leger,A. A newcomponentof theinterstellar matter:smallgrainsandlarge aromatic molecules. Annu. Rev. Astron. Astrophys. 27, 161198 (1989).

F

(

1013Wm

2)

F

(

1013Wm

2)

Wavelength (m)

a

8-m plateau

12-m plateau

V2361 Cygni (2005 October 19)

b

8-m plateau8-m plateau

12-m plateau

17-m plateau

V2362 Cygni (2007 June 22)

3

2

1

05 10 15 20 25 30

3

2

1

05 10 15 20 25 30

Figure 3 | Emergence of complex organics after nova outburst. a, b, Fits to

theSpitzer Infrared Spectrographspectra of novae V2361 Cygni (a) and V2362Cygni (b) 251 and 446 days after their respective outbursts. In addition to thegas emission line spectrum, both spectra have developed strong dust continua,and the 8- and 12-mm plateau features are clearly present. The continua ofV2361 Cygni and V2362 Cygni are fitted by modified blackbody intensities ofthe respective forms l20.2Bl(350K) and l

20.5Bl(365 K) (blue lines). Theorange lines are the8-, 12-and 17-mm plateau features and the solid blacklinesare discrete features at 5.3, 6.3, 6.9, 7.2 and 8.6 mm for V2361 Cygni and at 6.2,6.9, 7.6, 7.8 and 8.6 mm for V2362 Cygni. The observed spectra are shown assolid red lines and the fitted spectra are shown as dotted black lines. Thepresence of the 8- and 12-mm plateau features suggests that the aliphaticcomponent is the first to emerge after dust condensation. Because of the largenumber of emission lines, the atomic lines are not included in the fitting.

NNOO

SSSS

SSOO

OHOH

OO

OOH3CH3C

OO

NN

OO

SSOO

H3CH3C OOOO

NN

OOOHOH

NN

OO

CH3CH3

OO

OOFigure 2 | Proposed structure of thecarrier of UIEfeatures. Thestructure ischaracterized by a highly disorganized arrangement of small units of aromaticrings linked by different kinds of aliphatic chain. Other impurities such asoxygen, nitrogen and sulphur are also commonly present. This structurecontains about 100 carbon atoms and a typical nanoparticle may consist of

multiple structures similar to this one.

LETTER RESEARCH

0 0 M O N T H 2 0 1 1 | V O L 0 0 0 | N A T U R E | 3

Macmillan Publishers Limited. All rights reserved2011

8/3/2019 Nature 10542

4/4

6. Sellgren,K. The near-infrared continuum emission of visual reflection nebulae.Astrophys. J. 277, 623633 (1984).

7. Tielens, A. G. G. M. Interstellar polycyclic aromatic hydrocarbon molecules.Annu.Rev. Astron. Astrophys. 46, 289337 (2008).

8. Peeters,E. etal. Therich6to9 mm spectrumof interstellarPAHs.Astron. Astrophys.390, 10891113 (2002).

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, 810837 (2007).

10. Uchida, K. I., Sellgren, K., Werner, M. W. & Houdashelt, M. L. Infrared SpaceObservatory mid-infrared spectra of reflection nebulae. Astrophys. J. 530,817833 (2000).

11. Kwok,S., Volk,K. & Hrivnak,B. J. Chemicalevolutionof carbonaceous materialsinthe last stages of stellar evolution. Astron. Astrophys. 350, L35L38 (1999).

12. Onaka, T. Interstellardust: what do spaceobservations tellus?Adv. Space Res.25,21672176 (2000).

13. Clayton, G. C. et al. The role of polycyclic aromatic hydrocarbons in ultravioletextinction. I. Probing small molecular polycyclic aromatic hydrocarbons.Astrophys. J. 592, 947952 (2003).

14. Pendleton, Y. J. & Allamandola, L. J. The organic refractory material in the diffuseinterstellar medium: mid-infrared spectroscopic constraints. Astrophys. J. Suppl.Ser. 138, 7598 (2002).

15. Pilleri, P. et al. Search for corannulene (C20H10) in the RedRectangle. Mon. Not. R. Astron. Soc.397, 10531060 (2009).

16. Guillois, O., Nenner, I., Papoular, R. & Reynaud, C. Coal models for the infraredemission spectra of proto-planetary nebulae. Astrophys. J. 464, 810817 (1996).

17. Kahanpaa, J., Mattila, K., Lehtinen, K., Leinert,C. & Lemke, D. Unidentifiedinfraredbands in the interstellar medium across the Galaxy. Astron. Astrophys.405,9991012 (2003).

18. Sakata, A., Wada, S., Onaka, T. & Tokunaga, A. T. Infrared spectrum of quenchedcarbonaceous composite (QCC).II. A new identification of the 7.7and 8.6micron

unidentified infrared emission bands.Astrophys. J. 320, L63L67 (1987).19. Duley, W. W. & Hu,A. Polyynes andinterstellar carbon nanoparticles. Astrophys. J.

698, 808811 (2009).20. Jager, C., Huisken, F., Mutschke, H., Jansa, I. L. & Henning, T. H. Formation of

polycyclic aromatic hydrocarbons and carbonaceous solids in gas-phasecondensation experiments. Astrophys. J.696, 706712 (2009).

21. Duley, W. W. & Williams,D. A. Excitationof thearomatic infrared emissionbands:chemical energy in hydrogenated amorphouscarbon particles?Astrophys. J. 737,L44 (2011).

22. Papoular, R., Conrad, J., Giuliano, M., Kister, J. & Mille, G. A coal model for thecarriers of the unidentified IR bands. Astron. Astrophys. 217, 204208 (1989).

23. Papoular, R.The useof kerogendatain understandingthepropertiesand evolutionof interstellar carbonaceous dust.Astron. Astrophys. 378, 597607 (2001).

24. Kwok, S. The synthesis of organic and inorganic compounds in evolved stars.Nature 430, 985991 (2004).

25. Evans,A. etal. Infraredspectroscopyof novaCassiopeiae1993.IV. A closer lookatthe dust. Mon. Not. R. Astron. Soc. 360, 14831492 (2005).

26. Ney, E. P. & Hatfield, B. F. The isothermal dust condensation of Nova Vulpeculae1976. Astrophys. J. 219, L111L115 (1978).

27. Kwok, S.,Volk, K. & Bernath, P. On the originof infraredplateaufeatures in proto-planetary nebulae. Astrophys. J. 554, L87L90 (2001).

28. Derenne, S. & Robert, F. Model of molecular structure of the insoluble organicmatter isolated from Murchison meteorite. Meteorit. Planet. Sci. 45, 14611475(2010).

29. Cody, G. D. et al. Establishing a molecular relationship between chrondritic andcometary organic solids. Proc. Natl Acad. Sci. USA advance online publication,http://dx.doi.org/10.1073/pnas.1015913108 (4 April 2011).

30. Sellgren,K., Uchida, K. I. & Werner, M. W. The 1520mm Spitzer spectra ofinterstellar emissionfeatures in NGC7023.Astrophys. J. 659, 13381351 (2007).

Acknowledgements We thank A. Tang for technical assistance in the preparation ofthis manuscript.Thiswork wassupported by a grant to S.K. from the ResearchGrantsCouncilof theHongKong SpecialAdministrativeRegion,China(projectno.HKU 7027/11P).

Author Contributions S.K.designed theresearchand wrotethe paper. Y.Z.performed

data analysis and model fitting.Author Information Reprints and permissions information is available atwww.nature.com/reprints . The authors declare no competing financial interests.Readers are welcome to comment on the online version of this article atwww.nature.com/nature. Correspondence and requests for materials should beaddressed to S.K. (sunkwok@hku.hk).

RESEARCH LETTER

4 | N A T U R E | V O L 0 0 0 | 0 0 M O N T H 2 0 1 1

Macmillan Publishers Limited All rights reserved2011

http://dx.doi.org/10.1073/pnas.1015913108http://www.nature.com/reprintshttp://www.nature.com/naturemailto:sunkwok@hku.hkmailto:sunkwok@hku.hkhttp://www.nature.com/naturehttp://www.nature.com/reprintshttp://dx.doi.org/10.1073/pnas.1015913108