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Volume 386, Number 1, April IV 2002
Page(s) 296 - 307
Section Diffuse matter in space
DOI http://dx.doi.org/10.1051/0004-6361:20020125

A&A 386, 296-307 (2002)
DOI: 10.1051/0004-6361:20020125

Extinction calculations of multi-sphere polycrystalline graphitic clusters

A comparison with the 2175 Å peak and between a rigorous solution and discrete-dipole approximations
A. C. Andersen1, J. A. Sotelo2, 3, V. N. Pustovit4, 3 and G. A. Niklasson3

1  Department of Astronomy & Space Physics, Uppsala University, PO Box 515, 751 20 Uppsala, Sweden
    e-mail: anja@astro.uu.se
2  Dpto. de Fisica, Informatica y Matematicas, Universidad Peruana Cayetano Heredia, Aptdo. 4314, Lima, Peru
    e-mail: jsotelo@upch.edu.pe
3  Department of Materials Science, Uppsala University, PO Box 534, 751 21 Uppsala, Sweden
    e-mail: gunnar.niklasson@angstrom.uu.se
4  Institute of Surface Chemistry, NAS of Ukraine, 17 Gen. Naumova str., Kiev 03164, Ukraine
    e-mail: pustovit.vitaly@angstrom.uu.se

(Received 16 October 2001 / Accepted 22 January 2002 )

Certain dust particles in space are expected to appear as clusters of individual grains. The morphology of these clusters could be fractal or compact. In this paper we study the extinction by compact and fractal polycrystalline graphitic clusters consisting of touching identical spheres, based on the dielectric function of graphite from Draine & Lee (1984). We compare three general methods for computing the extinction of the clusters in the wavelength range $0.1{-}100~\mu$m, namely, a rigorous solution (Gérardy & Ausloos 1982) and two different discrete-dipole approximation methods - MarCODES (Markel 1998) and DDSCAT (Draine & Flatau 1994). We consider clusters of N= 4, 7, 8, 27, 32, 49, 108 and 343 particles of radii either 10 nm or 50 nm, arranged in three different geometries: open fractal (dimension D = 1.77), simple cubic and face-centred cubic. The rigorous solution shows that the extinction of the fractal clusters, with $N \le 50$ and particle radii 10 nm, displays a peak within 2% of the location of the observed interstellar extinction peak at ~4.6 $\mu$m -1; the smaller the cluster, the closer its peak gets to this value. By contrast, the peak in the extinction of the more compact clusters lie more than 4% from 4.6 $\mu$m -1. At short wavelengths ( 0.1-0.5  $\mu$m), all the methods show that fractal clusters have markedly different extinction from those of non-fractal clusters. At wavelengths >5 $\mu$m, the rigorous solution indicates that the extinction from fractal and compact clusters are of the same order of magnitude. It was only possible to compute fully converged results of the rigorous solution for the smaller clusters, due to computational limitations, however, we find that both discrete-dipole approximation methods overestimate the computed extinction of the smaller fractal clusters.

Key words: methods: numerical -- scattering -- dust, extinction -- ISM: general

Offprint request: A. C. Andersen, anja@astro.uu.se

© ESO 2002