Extinction calculations of multi-sphere polycrystalline graphitic clusters. A comparison with the 2175 å peak and between a rigorous solution and discrete-dipole approximations
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Extinction calculations of multi-sphere polycrystalline graphitic clusters. A comparison with the 2175 å peak and between a rigorous solution and discrete-dipole approximations. / Andersen, A. C.; Sotelo, J. A.; Pustovit, V. N.; Niklasson, G. A.
In: Astronomy and Astrophysics, Vol. 386, No. 1, 04.2002, p. 296-307.Research output: Contribution to journal › Journal article › Research › peer-review
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T1 - Extinction calculations of multi-sphere polycrystalline graphitic clusters. A comparison with the 2175 å peak and between a rigorous solution and discrete-dipole approximations
AU - Andersen, A. C.
AU - Sotelo, J. A.
AU - Pustovit, V. N.
AU - Niklasson, G. A.
PY - 2002/4
Y1 - 2002/4
N2 - 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 μm, 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 ≤ 50 and particle radii 10 nm, displays a peak within 2% of the location of the observed interstellar extinction peak at ∼4.6 μm-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 μm-1. At short wavelengths (0.1-0.5 μm), all the methods show that fractal clusters have markedly different extinction from those of non-fractal clusters. At wavelengths >5 μm, 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.
AB - 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 μm, 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 ≤ 50 and particle radii 10 nm, displays a peak within 2% of the location of the observed interstellar extinction peak at ∼4.6 μm-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 μm-1. At short wavelengths (0.1-0.5 μm), all the methods show that fractal clusters have markedly different extinction from those of non-fractal clusters. At wavelengths >5 μm, 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.
KW - Dust
KW - Extinction
KW - ISM: general
KW - Methods: numerical
KW - Scattering
UR - http://www.scopus.com/inward/record.url?scp=0036532170&partnerID=8YFLogxK
U2 - 10.1051/0004-6361:20020125
DO - 10.1051/0004-6361:20020125
M3 - Journal article
AN - SCOPUS:0036532170
VL - 386
SP - 296
EP - 307
JO - Astronomy & Astrophysics
JF - Astronomy & Astrophysics
SN - 0004-6361
IS - 1
ER -
ID: 232623361