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Issue 14 | December 2014
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Dear Colleagues,
Welcome to the December release of AstroPAH with which we are closing a verysuccessful year of 2014 reporting on multiple exciting topics. In light of the ground-breaking Rosetta mission, landing for the first time on a comet, this issue of AstroPAHfeatures meteorites in our Picture of the Month and the In Focus. The Picture of theMonth shows a beautiful carbonaceous meteorite. Conel Alexander, from the CarnegieInstitution for Science (USA), has written an excellent review on PAHs in meteoritesespecially for In Focus. Do not miss it!
Further exciting news come from the Laboratory Astrophysics Division (LAD) an-nouncing the awarding of its inaugural Laboratory Astrophysics Prize to Dr. LouisAllamandola of NASA’s Ames Research Center, California. Lou, our heartfelt congrat-ulations!
You will find in this issue of AstroPAH abstracts on the first experimental evidence forthe formation of fullerenes from PAHs, on evidence in astronomical observations for fullydehydrogenated PAHs, on the viability of the PAH model, high-quality thermochemistrydata on PAHs, and on the tentative detection of several new diffuse interstellar bands aswell as first tentative evidence for diffuse circumstellar bands in fullerene-rich environ-ments.
The editorial team will have a holiday break in January and AstroPAH will be back inFebruary with its next release. We thank you all for your contributions and please keepthem coming!
For those taking a break too, we wish you happy holidays!
The Editorial Team
Next issue: 17 February 2015.Submission deadline: 6 February 2015.
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AstroPAH NewsletterEDITORIAL BOARD:
Editor-in-ChiefProf. Alexander TielensLeiden Observatory (The Netherlands)
Executive EditorDr. Isabel AlemanLeiden Observatory (The Netherlands)
EditorsDr. Alessandra CandianLeiden Observatory (The Netherlands)
Dr. Elisabetta MicelottaInstitut d’Astrophysique SpatialeCNRS/Universite Paris-Sud (France)
Dr. Annemieke PetrignaniLeiden Observatory andRadboud University Nijmegen(The Netherlands)
Dr. Ella Sciamma-O’BrienNASA Ames Research Center (USA)
.Contents
PAH Picture of the Month 1
Editorial 2
In Focus: PAHs in Meteorites 4
Recent Papers 10
PAH Picture of the Month
Thin section of meteorite NWA 5958 as seenin cross-polarized light. NWA 5958 is a car-bonaceous chondrite of 286 g of mass thatwas found in Morocco in 2009.
Credits: John Kashuba, www.meteorite-times.com
CLICK HERE TOSUBSCRIBE TO AstroPAH
CONTRIBUTE TO AstroPAH!http://astropah-news.strw.leidenuniv.nl
Design by Isabel Aleman
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PAHs in Meteorites - A Review
C. M. O’D. Alexander
Introduction
Figure 1 - Cross section of a chondritic me-teorite. The millimeter sized spherical objectsare chondrules. The organic matter is foundin the dark, fine-grained matrix between thechondrules.
The chondritic meteorites (cf. Figure 1) arefragments of main belt asteroids that formedwithin 2-4 Myr of the formation of the Solar Sys-tem from the dust in the solar protoplanetarydisk (solar nebula). Since then, the most primi-tive chondrites have remained relatively unmodi-fied, although all have seen mild aqueous alter-ation and/or thermal metamorphism. While thedust from which they formed is predominantly So-lar System in origin, the chondrites did accretesome presolar material derived from the proto-solar molecular cloud. These include 10s ofmicron- to nanometer-sized circumstellar grains(SiC, Si3N4, Al2O3, MgAl2O4, CaAl12O18, TiO2,MgCr2O4, graphite, diamond, and crystalline andamorphous silicates) that formed around RGBand AGB stars, novae and supernovae1. Re-cently, it has been shown that it is unlikely thatlarge D enrichments can be generated in waterin disks2. Hence, the fact that the water in chon-drites is significantly enriched relative to the bulksolar D/H ratio3 indicates that some interstellar water survived Solar System formation as well2.
Since organic matter is almost certainly associated with water ice in molecular clouds, itseems likely that some interstellar organic matter has survived in meteorites. Chondritic me-teorites do in fact contain up to ∼3 wt.% organic C that is composed of a dominant macro-molecular, solvent insoluble organic material (IOM) and a less abundant but complex suite ofsolvent soluble organic compounds. The solvent soluble material includes amino acids, car-boxylic acids, nucleic acids, and, most relevantly here, small PAHs4−6 for simplicity here I willconsider PAHs to be any molecules that include one or more benzene rings. The IOM in eventhe least heated chondrites is composed of ∼50% aromatic C7 and is present in carbonaceous
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particles that range in size from several microns down to the resolutions of the various tech-niques used (typically 10s of nm)8−11. Unlike in the classical picture of carbonaceous grainformation in the interstellar medium (ISM), the meteoritic grains do not seem to be rims onmore refractory materials8.
Terrestrial contamination is a constant concern when studying the organic matter in mete-orites, but multiple lines of evidence, including large D and 15N enrichments, indicate that in themost pristine samples most of the soluble material12−14 and all the IOM15,16 is extraterrestrialin origin. Traditionally, the D and 15N enrichments have been viewed as clear evidence for in-terstellar origins for much or all of the organic matter (or its precursors). However, there is agrowing appreciation that conditions in parts of the outer Solar System may have resembledthose in molecular clouds and, therefore, that the D and 15N enrichments are not unambiguousindicators of an interstellar inheritance.
Figure 2 - NMR spectra of the insol-uble residues derived from EET92042(CR2), Orgueil (CII), Murchinson(CM2), and Tagish Lake meteorites.The spectral regions corresponding tovarious organic functional groups arenoted at the top. (modified from Cody& Alexander 2005).7
Here I review the nature of the PAHs in the solubleand insoluble material, discuss the evidence for andagainst their being interstellar, and compare them towhat is known of PAHs in the ISM. Numerous tech-niques have been applied to the study of the solubleand insoluble components. Probably the most com-mon is gas chromatography coupled with mass spec-trometry to identify individual compounds (GCMS) orto measure their isotope ratios (GCIRMS). Since thistechnique can only be used on volatile compounds, tostudy the IOM requires that it be broken down thermallyand/or chemically first. An alternative non-destructivetechnique for studying the IOM is nuclear magnetic res-onance (NMR) spectroscopy, which provides an aver-age picture of the functional group chemistry.
PAHs in the Insoluble Material
Determining the functional group chemistry of com-plex macromolecular materials like IOM is a consid-erable analytical challenge. The degradative productsstudied by GCMS and GCIRMS are dominated by smallPAHs (1-3 ring) that are often multiply substituted byshort aliphatic groups6,17−22. The largest PAH that hasbeen reported from IOM is coronene23,24. Measure-ment of the C isotopes of PAHs liberated from IOM fromthree different meteorites revealed conflicting behaviorwith C number19, perhaps because for the more labilecomponents the effects of parent body processes are superimposed on any isotope fractiona-tions associated with IOM synthesis. However, low or undetermined yields mean that it is notclear how representative the results of any of these degradative experiments are. For instance,during simple pyrolysis of IOM in He, at least 50% of the C forms a char and other C reactswith the O in the IOM to form CO and CO2. Also, difficulties with quantification mean that thesestudies provide, at best, a qualitative picture of the aromatic molecular constituents of IOM.
Non-destructive NMR spectroscopy, on the other hand, can detect all of the C and is relativelystraightforward to quantify. NMR studies of the most primitive IOM show it to be composedof ∼50% aromatic C in small, highly substituted PAHs7,25,26. Figure 2 shows the diversity ofIOM structures observed in four different meteorites that have experienced different degrees of
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parent body processing. Each spectrum is dominated by an intense spectral band centered at129 ppm that corresponds to the presence of aromatic (and possibly olefinic) carbon (both Hand C substituted). The extent of substitution on the PAHs is uncertain, but in IOM from themost primitive chondrites only ∼30% of the aromatic C is directly bonded to H. Since the PAHsin IOM are small, the average degree of substitution is probably of the order of 40-70%. Sucha high degree of substitution is not too surprising since a macromolecular material requiresconsiderable crosslinking between its constituents. The substituting moieties (side chains andcrosslinks) include short, highly branched aliphatic material, furans and ethers7,21,25−27.
PAHs in the Soluble Material
More than 70 free PAHs from benzene to as large as benzopyrene and benzoperylene havebeen identified in chondrites, many with aliphatic, OH and carboxyl substitutions28−33. The con-centrations of these PAHs in the bulk chondrites are typically a few parts per million or less.Pyridines and derivatives of phthalic acid have also been reported34. Compound specific Cisotopic measurements of two fairly primitive chondrites show that 12C/13C ratios decrease by∼2.5 from C6 to ∼C12 and then increase again by a similar amount up to ∼C17,28,35
20 . There isalso a suggestion of two formation pathways for the heavier PAHs35, but in both cases 12C isfavored over 13C with increasing C number. The size and systematic nature of the isotopic vari-ations amongst the heavier suggests kinetic isotope fractionation associated with cyclization orC addition at low temperatures in the ISM or Solar System. The decrease in 12C/13C ratios withincreasing C number amongst the C6-C12 compounds could be indicative of cracking, possiblybecause they were at least partly derived from the IOM.
The Solar vs. Interstellar Debate
There is evidence that IOM-like material was widely distributed in the early Solar System.Isotopically similar material is found in both the most primitive meteorites36 and in interplan-etary dust particles (IDPs)37 that probably have both cometary and asteroidal sources. Whenanalyzed at micron to sub-micron scales, this organic material exhibits tremendous isotopic het-erogeneity - in so-called hotspots, D enrichments as high as 40-50 times the terrestrial valueand 15N enrichments up to 3-4 times the terrestrial value have been reported. It should benoted that there is no simple correlation between the H and N isotope anomalies. Carbon iso-tope hotspots are rarer and the isotopic variations much more subdued (up to 20-30%), andagain do not correlate with anomalies in the other elements38. Strengthening the link betweencometary and meteoritic organics is the fact that the most primitive IOM15 and the refractory car-bonaceous dust from comet Halley39 have very similar elemental compositions (C100H79N4O15S3
and C100H80N4O20S2, respectively).
Other than the isotopes, perhaps the best evidence for the IOM being of interstellar origin isthe strong similarity between the IR absorption spectra in the 3-4 µm aliphatic C-H stretchingregion of primitive IOM and carbonaceous dust in the diffuse interstellar medium (DISM)40−42.Both appear to have aliphatic material that is dominated by short, highly branched chains.However, there are differences, the most notable amongst them being the apparent lack of Oand N heteroatoms in the diffuse ISM dust40. Pendleton and Allamandola40 also argued that thediffuse ISM dust is dominated by large PAHs unlike the IOM. These differences could perhapsbe reconciled if when the DISM dust is incorporated into cold molecular clouds and coatedin ice, the large PAHs are damaged by cosmic rays. As the grains are warmed during SolarSystem formation, rather than anneal the damaged regions in the large PAHs react with H-, O-and N-bearing radicals in the ice (H, OH, NH2, etc.), creating small islands of undamaged PAH
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surrounded and linked by damaged PAH that has been transformed into highly branched andheteroatom-rich aliphatic material43. This model has the added attraction that both the waterand other components of the ices are predicted to be D- and 15N-rich, and would naturallyexplain why the isotopically anomalous material seems to be primarily in the aliphatic material.Dust returned to the DISM, rather than incorporated into a forming star system, would have tolose its N and O functionality, perhaps by UV irradiation.
On the other hand, others have argued that the dust in the DISM resembles hydrogenatedamorphous C (HAC)44−47 or particles with functional group chemistries that at least qualitativelyresembles the IOM48. Indeed, the NMR spectra of some HACs do resemble those of primitiveIOM49. The O and N heteroatoms, as well as the D and 15N enrichments, could still be addedto this DISM dust by irradiation in molecular clouds.
Free PAHs and carbonaceous dust in the ISM are generally thought to have formed aroundC-stars40,48,50, the same types of star that many of the circumstellar grains in meteorites comefrom. Stellar models and observations predict, and the circumstellar grains confirm, that C-starsproduce dust with C and N isotopes that are far from the solar values 12C/13C ratios in C-starcircumstellar SiC and graphite grains range over roughly three orders of magnitude, and their Nisotopes are strongly depleted in 15N. Other sources of carbonaceous dust, such as supernovaeand novae, also produce isotopically highly anomalous dust. Yet in bulk and when analyzed atsimilar scales to the circumstellar grains (≥100 nm), the IOM is essentially solar in its C isotopiccomposition, and is solar or enriched in 15N. Also, the H in evolved stars should be essentiallydevoid of D, but there is little evidence in IOM for highly D-depleted regions. Hence, if the IOMand free PAHs have interstellar provenances, where the dust and PAHs in the ISM form willhave to be rethought. The C isotopes of IOM in particular would require that the carbonaceousdust from evolved stars be destroyed in the ISM relatively rapidly, and that most of the ISM dustformed in the ISM with a relatively homogenous isotopic composition that reflected the bulkC isotopic composition of the ISM. Predictions of carbonaceous grain lifetimes in the ISM areshort compared to dust production rates by stars, apparently requiring an efficient mechanismfor making carbonaceous grains in the ISM51.
However, there is still no unambiguous evidence that the organic matter in chondritic mete-orites (and IDPs) formed in the ISM, leading some to suggest that it may have formed in theSolar System. Much of the soluble organic matter may not be products of the ISM but mayformed from interstellar precursors amino acid synthesis, for instance, may have occurred byStrecker-cyanohydrin synthesis and Michael addition reactions in the chondrite parent bodies.The same may be true for the IOM. Cody, et al. 26 have suggested that the IOM formed by thespontaneous polymerization of interstellar formaldehyde from solution when ISM ices melted inthe chondrite parent bodies. To date, this model has been the most successful at reproducingthe functional group chemistry of IOM and some of the morphologies of IOM grains, althoughthe formose polymer tends to be too O-rich even after heating and it is not clear if there wouldbe enough formaldehyde in ISM ices since up to ∼10% of cosmic C is tied up in the IOM15.
Others have suggested that the organics in chondrites are entirely Solar System in origin.In this case, the IOM might have formed by autocatalytic processes at fairly high temperaturesin the inner Solar System52 and then reacted with H2D+ at high altitudes in the disk53,54, or thesoluble organics and IOM formed by irradiation of icy grains as they were transported throughoutthe disk by turbulence55.
In conclusion, primitive chondritic meteorites contain a significant amount of organic mate-rial, mostly in micron to submicron grains composed of a PAH-rich macromolecular material.Abundances of similar material in IDPs and comets are even higher. Hence, this material wasprobably very widely distributed in the solar nebula. The isotopic compositions of the organicmaterial suggest that it or its precursors formed via chemistry in very cold environments, pre-sumably in either the protosolar molecular cloud and/or the Solar System. If it formed in the
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protosolar molecular cloud, it affords a unique opportunity for understanding carbonaceous dustformation in the ISM. If it formed in the Solar System, it provides important constraints on con-ditions and transport in the solar nebula. However it formed, the organic material in meteoritesis of considerable astrobiological interest. Finally, if the organics in chondrites is representativeof organics in protoplanetary disks, when heated and/or irradiated it is a potential source of thePAHs observed in some disks56.
Ralf Kaiser is a Professor in the Department of Chemistry of th. RalfKaiser is a Professor in the Department of Chemistry of th. Ralf Kaiseris a Professor in the Department of Chemistry of th. Ralf Kaiser is aProfessor in the Department of Chemistry of th. Ralf Kaiser is a Professorin the Department of Chemistry of th.Conel M. O’D. Alexander is a research scientist in the Department ofTerrestrial Magnetism at Carnegie Institution for Science in WashingtonDC, USA. E-mail: [email protected].
Ralf Kaiser is a Professor in the Department of Chemistry of th. RalfKaiser is a Professor in the Department of Chemistry of th.
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1. E. Zinner, in Meteorites and Cosmochemical Processes Vol. 1 Treatise on Geochemistry (2nd Ed.)(ed A. M. Davis) Ch. 1.4, 181-213 (Elsevier, 2014).2. L. I. Cleeves et al., Science 345, 1590-1593 (2014).3. C. M. O. D. Alexander et al., Science 337, 721-723 (2012).4. Z. Martins & M. Sephton, in Origins and synthesis of amino acids Vol. 1 Amino acids, peptides andproteins in organic chemistry. (ed A.B. Hughes) 3-42 (Wiley-VCH, 2009).5. I. Gilmour, in Meteorites, Comets and Planets Vol. 1 Treatise on Geochemistry (ed A.M. Davis)269-290 (Elsevier-Pergamon, 2003).6. M. A. Sephton, Geochim. Cosmochim. Acta 107, 231-241 (2013).7. G. D. Cody & C. M. O. D. Alexander, Geochim. Cosmochim. Acta 69, 1085-1097 (2005).8. L. Remusat et al., Astrophys. J. 713, 1048-1058 (2010).9. C. Le Guillou et al., Geochim. Cosmochim. Acta 131, 368-392 (2014).10. C. Le Guillou & A. Brearley, Geochim. Cosmochim. Acta 131, 344-367 (2014).11. C. Floss et al., Geochim. Cosmochim. Acta 139, 1-25 (2014).12. S. Pizzarello & Y. Huang, Geochim. Cosmochim. Acta 69, 599-605 (2005).13. S. Pizzarello & W. Holmes, Geochim. Cosmochim. Acta 73, 2150-2162 (2009).14. Y. Huang et al., Geochim. Cosmochim. Acta 69, 1073-1084 (2005).15. C. M. O. D. Alexander et al., Geochim. Cosmochim. Acta 71, 4380-4403 (2007).16. C. M. O. D. Alexander et al., Geochim. Cosmochim. Acta 74, 4417-4437 (2010).17. M. A. Sephton et al., Geochim. Cosmochim. Acta 62, 1821-1828 (1998).18. M. A. Sephton et al., Planet. Space Sci. 47, 181-187 (1999).19. M. A. Sephton & I. Gilmour, Astrophys. J. 540, 588-591 (2000).20. M. A. Sephton et al., Planet. Space Sci. 53, 1280-1286 (2005).21. R. Hayatsu et al., Geochim. Cosmochim. Acta 41, 1325-1339 (1977).22. M. A. Sephton et al., Geochim. Cosmochim. Acta 64, 321-328 (2000).23. L. J. Kovalenko et al., Anal. Chem. 64, 682-690 (1992).24. M. A. Sephton et al., Geochim. Cosmochim. Acta 68, 1385-1393 (2004).25. G. D. Cody et al., Geochim. Cosmochim. Acta 66, 1851-1865 (2002).26. G. D. Cody et al., Proc. Nat. Acad. Sci. 108, 19171-19176 (2011).27. R. Hayatsu et al., Science 207, 1202-1204 (1980).28. I. Gilmour & C. T. Pillinger, Mon. Not. Roy. Astron. Soc. 269, 235-240 (1994).29. A. Shimoyama et al., Geochem. J. 23, 181-193 (1989).30. H. Naraoka et al., Chem. Lett. 17, 831-834 (1988).31. M. R. Wing & J. L. Bada, Geochim. Cosmochim. Acta 55, 2937-2942 (1991).32. M. A. Sephton et al., Planet. Space Sci. 49, 101-106 (2001).
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33. B. P. Basile et al., Organic Geochemistry 5, 211-216 (1984).34. S. Pizzarello et al., Science 293, 2236-2239 (2001).35. H. Naraoka et al., Earth Planet. Sci. Lett. 184, 1-7 (2000).36. H. Busemann et al., Science 312, 727-730 (2006).37. S. Messenger, Nature 404, 968-971 (2000).38. C. Floss & F. J. Stadermann, Astrophys. J. 697, 1242-1255 (2009).39. J. Kissel & F. R. Krueger, Nature 326, 755-760 (1987).40. Y. J. Pendleton & L. J. Allamandola, Astrophys. J. Suppl. 138, 75-98 (2002).41. Y. J. Pendleton et al., Astrophys. J. 437, 683-696 (1994).42. P. Ehrenfreund et al., Astron. Astrophys. 252, 712-717 (1991).43. C. M. O. D. Alexander et al., in Organic Matter in Space. (eds S. Kwok & S. A. Sandford) 293-297(Cambridge University Press, 2008).44. E. Dartois et al., Astron. Astrophys. 423, L33-L36 (2004).45. A. P. Jones et al., Quart. J. Roy. Astron. Soc. 31, 567-582 (1990).46. W. W. Duley & A. Hu, Astrophys. J. 761, 115 (2012).47. A. P. Jones et al., Astron. Astrophys. 558, A62 (2013).48. S. Kwok & Y. Zhang, Nature 479, 80-83 (2011).49. G. Cho et al., Journal of Applied Physics 104, 013531-013538 (2008).50. S. Kwok, Nature 430, 985-991 (2004).51. A. P. Jones & J. A. Nuth, Astron. Astrophys. 530, 44 (2011).52. J. A. Nuth, III et al., Astrophys. J. Lett. 673, L225-L228 (2008).53. L. Remusat et al., Earth Planet. Sci. Lett. 243, 15-25 (2006).54. L. Remusat et al., Astrophys. J. 698, 2087-2092 (2009).55. F. J. Ciesla & S. A. Sandford, Science 336, 452-454 (2012).56. V. C. Geers et al., Astron. Astrophys. 476, 279-289 (200
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On the viability of the PAH model as an explanation of theunidentified infrared emission featuresYong Zhang1 and Sun Kwok1
1 Space Astronomy Laboratory, Faculty of Science, The University of Hong Kong
Polycyclic aromatic hydrocarbon (PAH) molecules are widely considered as the preferredcandidate for the carrier of the unidentified infrared emission bands observed in the interstellarmedium and circumstellar envelopes. In this paper we report the result of fitting a variety of non-PAH spectra (silicates, hydrogenated amorphous carbon, coal and even artificial spectra) usingthe theoretical infrared spectra of PAHs from the NASA Ames PAH IR Spectroscopic Database.We show that these non-PAH spectra can be well fitted by PAH mixtures. This suggest that ageneral match between astronomical spectra and those of PAH mixtures does not necessarilyprovide definitive support for the PAH hypothesis.
E-mail: [email protected] for publication in ApJhttp://arxiv.org/abs/1410.6573
A sensitive spectral survey of interstellar features in thenear-UV [3050-3700A]N. H. Bhatt1 and J. Cami1,2
1 Department of Physics and Astronomy, The University of Western Ontario, London, ON N6A 3K7, Canada2 SETI Institute, 189 Bernardo Avenue, Suite 100, Mountain View, CA 94043, US
We present a comprehensive and sensitive unbiased survey of interstellar features in thenear-UV range (3050-3700 A). We combined a large number of VLT/UVES archival observa-tions of a sample of highly reddened early type stars – typical diffuse interstellar band (DIB)targets – and unreddened standards. We stacked the individual observations to obtain a red-dened “superspectrum” in the interstellar rest frame with a signal-to-noise (S/N) ratio exceeding1500. We compared this to the analogous geocentric and stellar rest frame superspectra as well
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as to an unreddened superspectrum to find interstellar absorption features. We find 30 knownfeatures (11 atomic and 19 molecular) and tentatively detect up to 7 new interstellar absorptionlines of unknown origin. Our survey is sensitive to narrow and weak features; telluric residualspreclude us from detecting broader features. For each sightline, we measured fundamentalparameters (radial velocities, line widths, and equivalent widths) of the detected interstellar fea-tures. We also revisit upper limits for the column densities of small, neutral polycyclic aromatichydrocarbon (PAH) molecules that have strong transitions in this wavelength range.
E-mail: [email protected]; [email protected] for publication in ApJShttp://arxiv.org/abs/1411.7700
Laboratory formation of fullerenes from PAHs: Top-downinterstellar chemistryJunfeng Zhen1,2, Pablo Castellanos 1,2, Daniel M. Paardekooper2,Harold Linnartz2 and Alexander G.G.M. Tielens1
1 Leiden Observatory, Leiden University, P.O. Box 9513, 2300 RA Leiden, The Netherlands2 Sackler Laboratory for Astrophysics, Leiden Observatory, Leiden University, P.O. Box 9513, 2300 RA Leiden, TheNetherlands
Interstellar molecules are thought tobuild up in the shielded environment ofmolecular clouds or in the envelope ofevolved stars. This follows many se-quential reaction steps of atoms and sim-ple molecules in the gas phase and/oron (icy) grain surfaces. However, thesechemical routes are highly inefficient forlarger species in the tenuous environ-ment of space as many steps are in-volved and, indeed, models fail to explainthe observed high abundances. This isdefinitely the case for the C60 fullerene,recently identified as one of the mostcomplex molecules in the interstellar medium. Observations have shown that, in some PDRs,its abundance increases close to strong UV-sources. In this letter we report laboratory find-ings in which C60 formation can be explained by characterizing the photochemical evolution oflarge PAHs. Sequential H losses lead to fully dehydrogenated PAHs and subsequent losses ofC2 units convert graphene into cages. Our results present for the first time experimental evi-dence that PAHs in excess of 60 C-atoms efficiently photo-isomerize to Buckminsterfullerene,C60. These laboratory studies also attest to the importance of top-down synthesis routes forchemical complexity in space.
11 AstroPAH - December 2014 | Issue 14
E-mail: [email protected] for publication in ApJLhttp://arxiv.org/abs/1411.7230
Press Release
Molecular striptease explains Buckyballs in space
Characterizing the infrared spectra of small, neutral, fullydehydrogenated PAHsC. J. Mackie1,2, E. Peeters1,3, C. W. Bauschlicher Jr4 and J. Cami1,3
1 Department of Physics and Astronomy, University of Western Ontario, London, ON N6A 3K7, Canada2 Leiden Observatory, Leiden University, PO Box 9513, NL-2300RA, Leiden, The Netherlands3 SETI Institute, 189 Bernardo Ave, Mountain View, CA 94043, United States4 NASA Ames Research Center, MS 245-6, Moffett Field, CA 94035, United States
We present the results of a computational study to investigate the infrared spectroscopicproperties of a large number of polycyclic aromatic hydrocarbon (PAH) molecules and theirfully dehydrogenated counterparts. We constructed a database of fully optimized geometriesfor PAHs that is complete for eight or fewer fused benzene rings, thus containing 1550 PAHsand 805 fully dehydrogenated aromatics. A large fraction of the species in our database haveclearly non-planar or curved geometries. For each species, we determined the frequenciesand intensities of their normal modes using density functional theory calculations. Whereasmost PAH spectra are fairly similar, the spectra of fully dehydrogenated aromatics are muchmore diverse. Nevertheless, these fully dehydrogenated species show characteristic emissionfeatures at 5.2 µm, 5.5 µm and 10.6 µm; at longer wavelengths, there is a forest of emissionfeatures in the 16–30 µm range that appears as a structured continuum, but with a clear peakcentered around 19 µm. We searched for these features in Spitzer-IRS spectra of variouspositions in the reflection nebula. We find a weak emission feature at 10.68 µm in all positionsexcept that closest to the central star. We also find evidence for a weak 19 µm feature at allpositions that is not likely due to C60. We interpret these features as tentative evidence for thepresence of a small population of fully dehydrogenated PAHs, and discuss our results in theframework of PAH photolysis and the formation of fullerenes.
E-mail: [email protected] for publication in ApJhttp://arxiv.org/abs/1412.0492
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A search of diffuse bands in fullerene planetary nebulae:evidence for diffuse circumstellar bandsJ. J. Dıaz-Luis1,2, D. A. Garcıa-Hernandez1,2, N. Kameswara Rao1,2,3,A. Manchado1,2,4 and F. Cataldo5,6
1 Instituto de Astrofısica de Canarias, C/ Via Lactea s/n, E-38205 La Laguna, Spain2Departamento de Astrofısica, Universidad de La Laguna (ULL), E-38206 La Laguna, Spain3 Indian Institute of Astrophysics, Bangalore 560034, India4 Consejo Superior de Investigaciones Cientıficas, Madrid, Spain5 INAF- Osservatorio Astrofisico di Catania, Via S. Sofia 78, Catania 95123, Italy6 Actinium Chemical Research srl, Via Casilina 1626/A, 00133 Rome, Italy
Large fullerenes and fullerene-based molecules have been proposed as carriers of diffuseinterstellar bands (DIBs). The recent detection of the most common fullerenes (C60 and C70)around some Planetary Nebulae (PNe) now enable us to study the DIBs towards fullerene-richspace environments. We search DIBs in the optical spectra towards three fullerene-containingPNe (Tc 1, M 1-20, and IC 418). Special attention is given to DIBs which are found to beunusually intense towards these fullerene sources. In particular, an unusually strong 4428 Aabsorption feature is a common charateristic to fullerene PNe. Similarly to Tc 1, the strongestoptical bands of neutral C60 are not detected towards IC 418. Our high-quality (S/N > 300)spectra for PN Tc 1 together with its large radial velocity permits us to search for the presenceof diffuse bands of circumstellar origin which we refer to as diffuse circumstellar bands (DCBs).We report the first tentative detection of two DCBs at 4428 and 5780 A in the fullerene-richcircumstellar environment around the PN Tc 1. Laboratory and theoretical studies of fullerenesin their multifarious manifestations (carbon onions, fullerene clusters, or even complex speciesformed by fullerenes and other molecules like PAHs, or metals) may help solve the mystery ofsome of the diffuse band carriers.
E-mail: [email protected]; [email protected] for publication in A&Ahttp://arxiv.org/abs/1411.7669
High-Quality Thermochemistry Data on Polycyclic AromaticHydrocarbons via Quantum ChemistryThomas C. Allison1 and Donald R. Burgess, Jr.1
1 Chemical Informatics Research Group, Material Measurement Laboratory, National Institute of Standards andTechnology, Gaithersburg, Maryland, USA
In this article, recent work on calculating high-quality enthalpies of formation for polycyclicaromatic hydrocarbons (PAHs) based on both density functional theory (DFT) and Gaussian-3(G3) model chemistry methods is discussed. It is shown that through the use of an empiricalcorrection model, the systematic errors in low-level DFT calculations can be controlled to pro-
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duce reliable thermochemistry. It is further shown that the G3 have a more regular systematicerror and as a consequence a simpler model may be used to correct for systematic deviations.It is seen that the resulting enthalpy of formation values are in good agreement with the avail-able experimental data, and that the predictions are sufficiently robust to point out certain errorsin experimental determinations. This work has been done as part of a larger effort to create acurated data set of property data for a large set of PAH molecules. In furtherance of this goal,UV/Vis spectra have been computed using time-dependent DFT. The collection of PAH data isbeing made publicly available through a web-based database that is briefly described.
E-mail: [email protected]. Aromat. Comp., 35, 16–31 (2015)http://www.tandfonline.com/doi/abs/10.1080/10406638.2014.892890#.VIW Lr6Rdbw
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