On the nature of interstellar carbonaceous dust

Planet. Space Sci., Vol. 43, Nos. 10111,pp.128771291, 1995

Pergamon

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On the nature of interstellar carbonaceous dust R. Papoular,‘,’ 0. Guillois,2 I. Nenner,2,3 J.-M. Perrin,4 C. Reynaud’ and J.-P. Sivan”

’ CEA, CE Saclay, Service d’Astrophysique, 91191 Gif-sur-Yvette, France ’ CEA, CE Saclay, Service des Photons, Atomes et Molecules, 91191 Gif-stir-Yvette, France 3 LURE, Laboratoire mixte CNRS, CEA et MRES, UniversitC Paris-&id, 91405 Orsay, France 4Laboratoire d’Astronomie Spatiale du CNRS, BP 8, 13376 Marseille Cedex 12, France Received 1 December 1994; revised 20 February 1995; accepted 6 March 1995

Abstract. The average galactic intersteliar extinction

cwve from 1000 to 8000 k can be synthesized using the measured, optical properties of standard demineralized coals with different degrees of graphitization (structural order). A highly-gra hitized polyycrystalline carbon simulates the 2175 ff hump and a weakly graphitized carbon simulates the underlying contirmum ; both contribute to the far-UV rise. An acceptable fit is obtained with,a grain radius distribution in aw3.’ and amax z 0.25 pm, Surface roughness is not critical. No spurious feature is introduced anywhere in the spectrum. The feature width increases with structural disorder and maximum grain size. These resnlts are understandable in the light of the known universal properties of coal.

jectured that it, too, could be carried by a coal-like material, albeit much less graphitized than anthracites. Here, we report similar measurements on the lesser evolved coals. A Kramers-Kronig analysis is applied to each reflectance spectrum obtained to date, which yields the complex index of refraction of the corresponding sample. We then go on to compute the corresponding average optical efficiency of grain populations with various size distributions [another improvement on Papoular et ul. (1993a), where only the Rayleigh-Gans limit was considered]. Extinction curves of mixtures of coals are also obtained. Comparison of these results leads us to propose a mixture of more or less graphitized coals as a model for each particular astrophysical environment. The possible contribution of silicates to the IS extinction curve @SEC) is briefly discussed.

1. Introduction 2. Coal definition and properties

In recent years, kerogens and coals have been shown to mimic the carriers of the Unidentified IR Bands (UIBs) (Papoular et al., 1989, 1993b; Guillois et al., 1994). Moreover, the study of carbonaceous chondrites strongly suggests that the organic component of primitive meteorites is mostly kerogen-like (Kerridge, 1990). The term kerogen designates the insoluble, three-dimensional, organic macromolecular skeleton which is the main constituent of lesser evolved coals. This prompted us to use coals as models of the carbonaceous components of interstellar (IS) dust. In a preliminary work (Papoular et al., 1993a), coals with carbon contents 390% (anthracites) were shown to be acceptable model carriers of the 2175 8, IS extinction feature, based on UV-visible reflectometry of various graphites and highly evolved coals. No modeling of the underlying continuum was offered, but it was conCorrespondence to : R. Papoular

The chemical and physical properties and the structure of coals and kerogens have been well studied and documented (Stach, 1986; Charcosset, 1990; Durand, 1980). The properties of coals are mostly functions of their age (i.e. mining depth) and depend very little on their geographical, botanical or zoological origin. Technically advanced countries have established parallel classifications of coals, ranging from the less evolved peats to the more evolved anthracites (Stach, 1986, p. 45) ; here, evolution is identined with ageing. Coal “banks” have been set up and can provide standard coals for each main evolutionary stage, together with detailed composition, calorific value and optical reflectance. Raw coals include up to 20% by weight of minerals (silicates, calcite, etc.) but our model of carbonaceous IS dust makes exclusive use of demineralized coals. When demineralized, coal is made up essentially of the three most abundant (chemically active) elements in the Universe, carbon, hydrogen

1288 and oxygen. A large fraction of the carbon is arranged m graphite-like “bricks” made of several layers of planar systems of benzene rings packed together in limited numbers. These so-called Basic Structural Units (BSUs) are oriented at random and linked by oxygen bridges, while most of the hydrogen atoms are bonded to individual peripheral carbon atoms. The evolution of coal is marked by the release of oxygen and hydrogen atoms and the subsequent aromatization (growth of the plane clusters of rings) and graphitization (ordering of the clusters parallel to each other), which are accelerated by temperature and decelerated by pressure. The motor of this process is the rc delocalized bond of carbon, for it provides a sizeable increase in bond energy per electron as the number of hexagonal rings per cluster increases (Robertson, 1988). This natural evolution is an essential clue to understanding the inception of carbonaceous grains in CS envelopes and their poly- and metamorphism in space. The functional groups formed by hydrogen and oxygen w-ith carbon atoms are responsible for the vibrational bands which mimic the UIBs. The graphite-like bricks give rise to the X-Z* resonance near 2200 A and the GCT* resonance near 800 A (Papoular et al., 1993a), both characteristic of the electronic structure of the carbon skeleton. The evolution of coal is marked by an increase of the E-Z* feature intensity and a decrease of its breadth. Also, the aromatic vibrational features are strengthened as compared with the aliphatic ones, while all vibrational features decrease in absolute intensity. The c-g* feature, on the other hand, is only weakly affected because the corresponding o-bond is not characteristic of graphitic order (Robertson, 1986) ; it is rather akin to the tetrahedral “diamond-like” bond. Coal also displays a very strong spectral continuum due to “free” electrons at short wavelengths and a much weaker continuum due to phonons in the IR. Both are consequences of the solid state of the material. The transition between them corresponds to a semiconductor band gap of order 1 eV. As aromatization proceeds, the strength of the IR continuum increases relative to the vibrational bands. Thus, coal provides a continuous range of optical properties from which to choose the best fit to celestial spectra of different circumstellar or interstellar origins. Our present contribution to this field has been to measure the optical properties of the standard coals from 300 to 9oooA.

3. Experimental method and data analysis

The reasons for having recourse to reflectometry and the ways to deduce the refractive index from the measurements are developed in Papoular et al. (1993a). Let us briefly recall here that, in the range 3004000 A, we used the synchrotron radiation from the positron storage ring Super AC0 at the LURE facility in Orsay (France). The coal samples were in the form of pressed pellets of fine demineralized powder. These measurements were complemented by diffuse reflectance measurements on the same samples, in the range 2000-9000 A, using a polytetrafluoroethylene-

coated imtegrating sphere mounted in a Caryl spectrophotometer. This set-up also delivers data in the range 20004000 8, ; they are similar to the synchrotron data in the same range, so that the spectra of both origins can be joined smoothly. The coals used in this study are the standard coals of the CERCHAR minibank set up by the French Coal Board and the Dutch Center for Coal Specimens (SBN). These coals are not industrial but channel samples coming straight from the mine. They are well characterized “standards” and their properties are described in Charcosset (1990). They are listed below by order of increasing rank (degree of graphitization) together with their international classification, mine name and abbreviation : (1) subbituminous, Provence (Pr) ; (2) high volatile bituminous, Vouters (Vo) ; (3) low volatile bituminous, Mericourt (Me) ; (4) semianthracite, Escarpelle (Es) ; (5) anthracite, La Mure (Mu). The raw coals were demineralized by Institut Francais du P&role and finally delivered in the form of fine powder. These powders were then pressed under lo-13 kbar into solid pellets which served as samples (except for La Mure coal, of which the powder was deposited in a horizontal cup in the vacuum chamber). Since the highest ranking standard coal in the minibank is still less evolved than the most graphitized coal that is known to exist [semi-graphite ; see Stach (1986, p. 46)], we extended the range of aromatizations by adding polycrystalline graphite (PG) of very high purity (nuclear reactor grade). This is made up of randomly oriented crystallites, 100-1000 8, in size. For calibration purposes, we also included HOPG (highly oriented pyrolytic graphite), the artificial carbon that comes nearest to crystalline graphite. Once the spectral reflectance of a sample is obtained, it is possible to deduce the corresponding complex refractive index by standard Kramer-s-Kronig’s analysis [see Papoular et al. (1993a)]. This is the principal source of error on the absolute values of the indices but reproduces the spectral profile reasonably well. Using these refractive indices and Mie’s theory, the average extinction efficiency, (QzXt), was then computed for spherical grains of radius a following a distribution in d and ranging from amln to a,,, with a fine binning. We assumed a weak optical density and neglected multiple scattering, so that (Q,,,) must be proportional to the IS extinction, A(i), of the corresponding dust. A range of parameter values has been explored : 0. l-l pm for amax. 20-100 8, for amin and - 4.5 to - 1.5 for p. Several porosity and roughness rates have also been considered, using the discrete dipole approximation (Perrin and Sivan, 1990).

4. Results

Figure 1 shows the spectral rehectances of graphites and coals ; the newly measured less graphitized coals confirm the trends observed previously: with increasing graphitization (accompanied by release of hydrogen and oxygen atoms), the n-n* bump (1600&3500 A) becomes more

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R. Papoular et al. : Interstellar carbonaceous dust ‘m

varies from 0.1 to 1 /lrn. Porosity broadens and red-shifts

the hump. Even as little as 15% porosity shifts the band unacceptably ; it seems, therefore, that the hump carriers in the IS medium are not porous. On the other hand, rugosity rates up to 20% do not perturb the extinction curves notably, which means that the latter are not very sensitive to grain shape. The albedo spans the range 0.2-

0.4 with a trough near 2000 A and peaks near 1500 and 3500 A. Comparison with similar work on hydrogenated

I

I

I

t

0.2

0.4

0.6

0.8

1 '

0.0

Wavelength (pm)

Fig. 1. Spectral reflectances of coals: (1) Pr (x1/10); (2) Vo (x1/5) ; (3) Me (x1/3) ; (4) Mu and graphites; (5) polycrystalline graphite (PG) ; (6) HOPG, in order of increasing aromaticity (i.e. size of benzene clusters)

and more pronounced, which translates into a stronger and narrower 2200 8, feature. Figure 2 displays the average optical extinction

efficiencies (Q&L)) of spherical grains for the more significant materials. In all cases, the grain radius dis-

tribution is in ae3.’ (Mathis et al., 1977) with a lower limit of 20 A, suggested by the average size of the BSUs. Note

amorphous carbon (Colangeli et al., 1993) suggests that its structure is similar to that of the less graphitized coals. Thermally altered quenched carbonaceous composites exhibit a stronger, but still wide, hump (Sakata et al., 1994), indicating an improved graphitization. Individual

polycyclic aromatic hydrocarbon (PAH) molecules small enough to undergo transient heating by UV photons exhibit sharp features in the visible-UV, as illustrated by Joblin et al. (1992, fig. 2). These authors have therefore sought to obtain mixtures (“soups”) of heavier, free-flying PAHs by strongly heating coal pitch. The carbon clusters evaporated in this way are indeed much heavier (170-500 a.m.u.) and the features are broader and overlap. The fit to the ISEC is still unacceptable, but the improvement shows that the IS dust must be made of still more complex and larger clusters, which points precisely to coal-like materials.

that the weaker and wider features of the less evolved

coals are associated with an earlier far-UV rise. Increasing the exponent p or the upper grain size limit leads to wider, shallower and red-shifted Z--71” peaks, as expected. In the case of crystalline graphite, this red-shift is more pro-

nounced and already occurs at small grain sizes because a surface resonance mode sets in (Frohlich resonance), which is one of the weaknesses of graphite as a dust model. This is not the case for coal or polycrystalline graphite, because their dielectric constants are never negative for any polarization, which is a consequence of their random

structures. For a distribution exponent of -3.5, the n--71* peak wavelength is restricted to the range 2168-2189 8, as amax

1

0.0

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0.2

0.4

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0.6

5. Comparison with IS extinction Examination of the (Q,,,) values of different coals naturally leads us to consider various combinations of these as a model for IS extinction. After trying a number of such combinations, we selected a mixture of 50% poly-

crystalline graphite and 50% Me coal, with amax = 0.25 pm and no artificial porosity, as the nearest model to the average galactic ISEC. This is illustrated in Fig. 3, where the (Qe,,(n)) of this mixture, calculated as in Section 3, is superposed upon an average ISEC proposed by Mathis and Whiffen (1989) as A(L)/&, and normalized to (Q& at 3, = 5470 A. Data and model agree to better than 10%. The fit is not sensitive to surface roughness nor to changes of p in the range [ - 3.9, - 3.31. It is degraded by 10% changes in composition.

t

0.8

Wavelength (pm) Fig. 2. Optical extinction efficiencies, (Q&L)), for various grain populations of: (1) Pr (x1/5) ; (2) Vo (x1/3) ; (3) Me (x1/2) ; (4) Mu; (5) PG; (6) HOPG. In all cases, aman = 2500 & amin = 20 8, ; size distribution in a-3.5

Fig. 3. The average galactic ISEC [crosses ; Mathis and Whiffen

(1989)] and our best fit (solid line) obtained by mixing equal amounts of PG and Me coal (see Fig. 2)

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Let us now estimate the quantity of carbon which muse be condensed in coal-like material in order to fit the average galactic ISEC. First recall that, for a grain size distribution with exponent -3.5, Q> = $fc,
O.“M

0.8

where urn,* = 20 A and amax = 0.25 pm and we have taken the dust density to be 1.3 and 1.7 g/cm3 for Me coal and polycrystalline graphite, respectively. Within errors, and uncertainty over amin, the result is about the upper limit of the available carbon in the solar vicinity (Meyer, 1988 ; Grevesse and Anders, 1989). Since it is considered that only N 70% of the carbon atoms are condensed into grains (Spitzer, 1978), at least 30% of A” should be ascribed to some other carrier. Silicates are obvious candidates since they must dominate the mid-IR extinction at least. Unfortunately, while it is clear, from the lo- and l&pm bands, that IS silicates must be amorphous, no refractive index is available for them in the visible-UV. Even the UV spectrum of the “astrophysical silicate” developed by Draine and Lee (1984) is based on crystalline silicate data and therefore exhibits a strong discontinuity in optical properties around 1. = 1500 A, which has no counterpart in the ISEC. Although this may be washed out by a suitable grain size distribution (Kim et al., 1994), we prefer, for the reasons given above, to postpone the addition of silicates to the model until more laboratory data are available. In any event, the goodness of the fit in Fig. 3 indicates that the spectral profile of the extinction contribution of the unknown material should be similar to that of Me coal (see Fig. 2). Both may even be mixed in the dust. A thorough interpretation of the observed variations of the ISEC with line of sight is outside the scope of this paper. A few hints can, however, be given. In the present model, the changes in the ISEC are not explained mainly by shape or size variations of the dust grains (although these play their part), but rather by changes of the bulk optical properties due to concomitant modifications of composition (H/C, O/C) and structure of the dust material, which can be mimicked by changing the composition of mixtures of more and less graphitized (evolved) coals. This is in complete agreement with the conclusions drawn by Draine and Malhotra (1993) from a study of possible causes of variations in the extinction profile. In the Earth, coal evolution occurs naturally at a fast pace because of underground warmth. In the diffuse IS medium, a parallel evolution may occur, albeit more slowly, under the annealing action of energetic photons from hot stars. This is illustrated in Fig. 4 by the LMC, which is younger than our Galaxy, and by S30 Dor, in the EMC, which is younger still : the younger the region, the less graphitized is the coal and the weaker is the hump. At the present stage, the accuracy of reflectance measurements and Kramers-Kronig analysis does not warrant

Wavelength (pnj Fig. 4. (a) S30 Dor in LMC: data (filled circles) and fit (line) with 25% PG and 75% Pr; LMC (530 Dor excluded) : data (open circles) and fit (line) with 50% PC and 50% Vo (both shifted upwards by 0.3)

a more elaborate fitting procedure, nor more complex combinations of coals. One defect of our semi-graphite coal (polycrystalline graphite) as a model of the hump carrier is the excessive width of its 2175 A feature: 1.7 pm-‘. This may be an experimental artifact due to the light scattering associated with reflection on the sample surface, which is not perfectly specular, or with the crystallites being too large. However, the defect could also be due to incomplete graphitization of our sample, for the main cause of widening of the 2200 A feature is known to be structural and chemical disorder (e.g. Robertson, 1986, 1988). Thermalmotion also widens electronic bands by creating states at their edges without perturbing their core [Cody, 1984). When all these effects have been quantified, it may well turn out that some of them can explain why the IS feature width varies while its peak wavelength remains nearly constant. For all the aforementioned deficiencies and queries, the quality of the fits in Figs 3 and 4 is good enough to convey the global message : highly graphitized coals can provide the bump (~-71” feature) in the ISEC, weakly aromatized coals can provide the underlying UV-visible continuum and both contribute to the far-UV rise (red wing of the g-c* feature) ; no spurious feature is introduced anywhere in the spectrum ; no fine tuning of grain size or shape is required. Tables of the complex index of refraction of graphites and coals are available on request. Acknowledgements. We are grateful to P. Martin (Orsay Uni-

versity and LURE) for help with the instruments attached to the light source, and to E. Campinchi for help with the computations and illustrations. References Charcosset, II. (ed.), Advanced h4ethodoCogie.s iti coaoal Churucterization. Elsevier, Amsterdam, 1990. Cody, G. D., Hydrogenated amorphous silicon, in Semiconductors and Semimetals (edited by J. Pankove), Vol. 21 B,

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