E. EPIFANI

E. PALOMBA

J. R. BRUCATO

L. COLANGELI

PALUMBO, Pasquale

ROTUNDI, Alessandra

V. MENNELLA

English

Archivio della ricerca - Università degli studi di Napoli &quot;Parthenope&quot;

Pergamon www.elsevier.nl/locate/asr Adv. Space Res. Vol. 24, No. 4, pp. 439442, 1999 b 1999 COSPAR. Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain 0273-l 177/99 $20.00 + 0.00 PII: so77.7.I 177(99)00084-x CARBONACEOUS GRAIN PROCESSING IN SPACE AND IN THE LABORATORY V. Mennella’, L. Colangeli’, J. R. Brucato’, E. Epifani’, E. Palomba’, P. Palumbo2, and A. Rotundi I Osservatorio Astronomico di Capodimonte, via Moiariello 16, 80131 Napoli, Italy ’ Istituto Universitario Navale, Via De Gasperi 5, 80133 Napoli, Italy ABSTRACT Laboratory simulations of carbonaceous grain processing which occurs in space are fundamental to outline an evolutionary pathway for these particles. We consider the UV spectral changes induced in hydrogenated carbon grains by thermal annealing, W irradiation and ion bombardment. The results give the opportunity to interpret observations in different space environments. In particular, modelling of the optical properties, based on a description of the electronic structure of carbons, indicates small hydrogenated amorphous carbon grains, with different degrees of UV irradiation, as the carrier of the ubiquitous W interstellar extinction bump. INTRODUCTION 01999 COSPAR. Published by Elsevier Science Ltd. It is well settled that carbonaceous grains represent an important component of circurnstellar and interstellar dust. Despite the wealth of observations many aspects of the nature of this component are not completely known. A typical example is the identification of the carriers of the strongest feature in the interstellar extinction curves, the absorption bump at 4.60 pm* (217.5 nm). From the observational point of view the bump has been studied in detail, for a review see Massa and Savage (1989). The band peak position, XO, is extremely stable, with variations < l%, along different lines of sight, while the band width, y, varies around 1.00 l& by 25 %. Small y values are associated with the diffuse cloud medium, while large widths are observed in dense quiescent regions (Fitzpatrick and Massa, 1986). There is no correlation between x0 and y, although the two extinction curves with the largest ~0 values show the widest bumps (Cardelli and Savage, 1988). A rather general consensus exists on the attribution of the feature to a carbon-based material. The ideal sequence of aromatic structure, from polycyclic aromatic hydrocarbons to graphite, has been considered, even if the exact nature of the carrier has not been identified. For many models, further difficulty arises from the revised abundance of carbon (Snow and Witt 1995). Recently, the influence of grain agglomeration on the W bump shape has been studied experimentally by Schnaiter et al. (1998). Modelling of the W bump has been carried out by focusing the attention mainly on variations of geometric factors (i.e. shape, size, particle coating) of specific materials such as graphite. However, there is no compelling reason why the dielectric properties of the carriers should remain the same everywhere in the interstellar medium (ISM). There is indeed increasing evidence that carbonaceous grain composition varies in response to W and cosmic ray irradiation, as well as to chemical interactions with the interstellar gas (e.g. Jones et al. 1990). The key point is whether these processes can give rise to the bump carrier. An evolutionary scenario for carbon grains has been outlined by Hecht (1986) and Sorrel1 (1990). They recognised the importance of hydrogen in the formation and evolution of these particles. The feature in the interstellar extinction is interpreted in terms of dehydrogenation and graphitization due to annealing caused by exposure to strong W radiation and cosmic ray bombardment. In this context, laboratory simulations can be very useful to check the validity of W bump modelling and the actual transformation of carbonaceous grains due to processing. CARBONACEOUS GRAIN PROCESSING IN THE LABORATORY 439 440 L! Mennella ef al. Specific laboratory experiments aimed at simulating grain processing which occurs in space and at studying the evolution of the W spectrum in hydrogenated carbon grains (ACH2) have been carried out (Mennella et al. 1995; 1996; 1997). In particular, thermal annealing, W irradiation and ion bombardment of ACH2 have been considered. At short wavelengths, the extinction spectrum of ACH2 shows a band falling at - 90 nm in the extreme-ultraviolet, while no evident feature is present in the W region. Details about the production method, morphological and optical properties of ACH2 are reported in Colangeli et al. (1995) and Rotundi et al. (1998). A W resonance, whose evolution depends on the specific mechanism, is activated after processing. For annealed ACH2, the W resonance shifts towards longer wavelengths, up to 259 nm at 800 “C, as the annealing temperature increases. On the other hand, W irradiated samples are characterised by a band stable at 215 nm, whose intensity increases with the W dose. Although the highest dose deposited in the grains (4~10~~ eV cme2) is 7 times smaller than that expected (DM = 3~10~~ eV cme2) during the typical time (3x10’ yr) that grains spend in the diffuse cloud medium before entering in the dense cloud medium (e.g. Jenniskens, 1993), this experiment suggests that a transformation by W photons of hydrogenated amorphous carbon grains into pure graphite particles is unlikely (Mennella et al., 1996). Also in the case of 3 keV He+ ion bombardment of ACH2, a process that simulates the interaction of cosmic rays with carbon grains in the ISM (Jenniskens et al., 1993), the W band shifts up to only 215 nm, after a dose of 660 eV/ C atom. However, the band is less intense and the time-scale necessary to produce this effect is larger than in W processing. Therefore, W irradiation is the dominant process for the transformation of hydrogenated carbon grains into the W interstellar bump carriers. It is worth noting that the capability of particle irradiation of destroying ordered aromatic structures on short time-scale poses the problem of the survival of graphite particles in the ISM. The results of these experiments indicate that W bump carriers should not have a marked graphitic character. They should rather be formed of small graphitic clusters and a sp3 component whose relative abundance depends on the residual hydrogenation and on the processing degree. To get insights into the dielectric properties of laboratory processed hydrogenated carbon grains a model for the dielectric function has been considered. It relies on the basic relation between the imaginary part of dielectric function, &i(E), and the electronic density of states for disordered materials (see Mennella et al. 1998 and references therein for further details). By assuming gaussian profiles for x and (T states, it is possible to derive the contribution of 7c - Z* and (T - (T* transitions to pi (E). The Kramers-Kronig relation is used to compute the real part of E(E). With the dielectric function defined in this way, the fits to laboratory data have been carried out by adopting the optical cross section per unit volume for a continuous distribution of ellipsoids, C(X) / V = 47txIm[&log(&) / (E - l>] , to account, in a first approximation, for the grain agglomeration effects on the optical properties (Rouleau and Martin, 1991). The best fit parameters indicate that W irradiation gives rise to an increase in cluster number and average size and a reduction of the size dispersion. On the other hand, only slight structural modifications take place after thermal annealing up to 415 “C, in agreement with previous results: a significant growth in size of the sp2 clusters occurs only after a complete hydrogen effusion (-600 “C). At 800 “C, the cluster average size is larger and the size dispersion is double than for the most W irradiated hydrogenated carbon grains. W and thermal processing induce different structural modifications in hydrogenated carbon grains, with W photons giving rise to a lower dispersion of the graphitic cluster size. A possible interpretation for this effect is a decrease of the dehydrogenation efficiency by W radiation that prevents larger graphitic clusters from a further dimensional growth. MODELLING OF THE W INTERSTELLAR BUMP The general validity of the dielectric function model and its successful test on laboratory data lead us to adopt it to investigate the nature of the W bump carbonaceous carriers in the ISM. We recall that, in a typical diffuse cloud time-scale of 3x10’ yr, the estimated W dose deposited in carbonaceous grains is DM = 3~10~~ eV cm-‘, while the W dose, D, varies from 0.0017 h to 0.13 DM for laboratory samples. To infer the dielectric properties for higher irradiation we take advantage of the linear dependence on log(D) of the best fit 7c parameters of the W irradiated analogues. We extrapolate this trend up to DM to represent the W processing pathway for interstellar hydrogenated carbon grains. Therefore, our working hypothesis to deal with the W interstellar bump is that the dielectric functions computed along this track can represent those of the processed carbon grains responsible for the feature in the ISM. These W processed hydrogenated carbon grains are labelled as Bump Carrier Carbons (BCC). Since the W bump carriers are expected to be small (e.g. Sorrell, 1990), we compute the W absorption profile of BCC spherical grains (see Figure 1) in the Rayleigh limit, C(x) / V = 6nxIm[(& - 1) / (E + 2)], to model the W Carbonaceous Grain Processing 441 BCC I: D=D, 1 BCC II: D=O.B7D, - BCC III: D-0.50D, CC I”: D=0.33D, _ C V: D=O.l3D, BCC VI: D=O.O3D, I , , , * 5 6 Fig. 1 Cross section per unit volume of BCC spherical grains as a function of the W dose expressed as a function of Dbl. The profiles BCC V and VI correspond to UV irradiated laboratory samples, while the other profiles refer to arbitrary reference fractions of D M. The vertical lines indicate the average and extreme values observed for the interstellar bump peak position. Inset: comparison between interstellar curves (dashed lines) and the profiles computed as a linear combination of BCCs + a linear background. The Seaton average bump and the profiles with the extreme deviations in xo and y are reported. For HD 29647 a combination of profiles BCC V and VI is sufficient to reproduce the bump. extinction bump. In particular, we attribute the interstellar bump to a linear combination of absorption (the profiles reported in Figure 1) from different BCC populations, as several interstellar regions with different physical conditions are generally sampled along a line of sight (Fitzpatrick and Massa, 1986). Of course, it needs to be verified whether the BCC model is able to explain the phenomenon it is proposed for and whether all the assumptions are physically correct and compatible with the dust environment. Concerning the first point, the BCC model is fully consistent with the observations, including the almost constant position of the absorption maximum with variations of the bump width. As shown in Figure 1, it is possible to reproduce not only an average profile, but also the interstellar profiles with the extreme deviations in peak position and band width. With reference to the carbon abundance constraint, it is worth noting that a C/H ratio of 48 ppm is needed to reproduce the Seaton average bump in our model, versus 55 ppm required with graphite (Mathis, 1996). Moreover, the model is compatible with the dust environment. The absorption from less processed BCC grains prevails for wider bumps, which are associated with denser regions, while more processed BCC particles dominate along diffuse cloud medium lines of sight which show sharper bumps. The observed bump sharpening from dense to diffuse clouds is mainly the result of a UV processing increase. In addition, we stress that, by using only the BCC V and BCC VI profiles, which have been computed with the optical constants of our laboratory analogues, two extinction bumps (Cardelli and Savage, 1988) - HD 62542 and HD 29647 reported in Figure 1 - are reproduced. Although these two bumps represent extreme cases in terms of intensity, peak position and width, a good match is obtained by using only laboratory analogue materials. As far as the physical correctness of our model is concerned, the point to be checked is the reliability of the extrapolated BCC. To do this, we computed the effective number, n,ff, of n: electrons per carbon atom contributing to the UV resonance; qff must be 5 1 (Robertson 1991). Actually, n+t, computed from the n contribution to the imaginary part of the dielectric function, is 0.45 for D = DM. This value is fully consistent with the values reported for other carbonaceous materials (Robertson, 1991). It should be noted that the BCC dielectric functions, even for D = DM, are significantly different from those of the <<astronomical graphite>> (Draine and Lee, 1984) or any graphitic carbon proposed as bump carrier up to date. Unlike previous predictions, the UV processed hydrogenated 442 V. Mennella et nl. carbon grains considered here do not have a marked graphitic character. The carbon atoms responsible for the UV absorption are organised in sp2 clusters which increase in number and tend to have a peaked size distribution as the UV dose increases. The value of the optical gap (0.65 eV for D = DM) corresponds to an aromatic coherence length of 12 A (Robertson 1991). The final point to be considered is the assumption of spherical grains to compute the BCC absorption profiles. This assumption is in line with most UV interstellar extinction feature models. Spheroids have been considered only for graphite, because small graphite spheres cannot provide a peak at 217.5 nm (e.g. Hecht, 1986; Draine and Malhotra, 1993). In the case of BCC, the assumption of spheres, while compatible with observations, is not tight: the match of the interstellar feature can be achieved even if spheroids (excluding only extreme shapes) are considered besides spheres (Metmella et al.1998). Unlike the graphite approach, where the dielectric function is fixed and variations of shape and/or other geometrical factors are invoked to reconcile that material with observations, in our model the contributions from different BCC dielectric functions can be combined to compensate for the peak shifts introduced by spheroids and to maintain the UV bump at the proper position. In conclusion, the proposed BCC model is able to reproduce the observational properties of the UV interstellar bump. The reason why this was possible is that we focussed the attention not on a single material but on a class, UV processed hydrogenated carbon grains, whose optical properties vary according to the processing degree in different region of the ISM. Different contributions of BCC particles along different lines of sight can produce the almost constant peak position associated with band width variations. ACNOWLEDGMENTS This work has been supported by ASI, MURST and CNR research contracts. REFERENCES Cardelli, J. A., and B. D. Savage, Astrophys. J., 325, 864 (1988). Colangeli, L., V. Mennella, P. Palumbo, A. Rotundi, and E. Bussoletti, Astrun. Astrophys. Supp2. Series, 113, 561 (1995). Draine, B. T., and H. M. Lee, Astrophys. 1. 285, 89 (1984). Draine, B. T., and S. Malhotra, Astrophys. J. 414, 632 (1993). Fitzpatrick, E. L., and D. Massa, Astruphys. J. 307,286 (1986). Hecht, J. H., Astrophys. J. 305,817 (1986). Jenniskens, P., Astrun. Astrophys. 274,653 (1993). Jenniskens, P., G. A. 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Bussoletti, Astron. Astrophys. 329, 1087 (1998). Rouleau, F., and P. G. Martin, Asrrophys. J. 377,526 (1991). Schnaiter, M., H. Mutschke, J. Dorschner, Th. Henning, and F. Salama, Astruphys. J. 498,486 (1998). Snow, T. P., and A. N. Witt, Science 270, 1455 (1995). Sorrell, W. H.; Mon. Not. R. Astr. Sot. 243,570 (1990). 

Carbonaceous grain processing in space and in the laboratory

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Carbonaceous grain processing in space and in the laboratory

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Archivio della ricerca - Università degli studi di Napoli "Parthenope"

Carbonaceous grain processing in space and in the laboratory

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Archivio della ricerca - Università degli studi di Napoli &quot;Parthenope&quot;

Archivio della ricerca - Università degli studi di Napoli "Parthenope"