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Structures Produced by Ballistic Agglomeration: BA, BAM1, and BAM2 Geometries

Bruce T. Draine, Dept. of Astrophysical Sciences, Princeton University

Prescriptions for producing random aggregates of spherules are discussed in Shen, Draine, and Johnson (2008) (arXiv:0801.1996 [astro-ph]), where a simple quantitative measure of "porosity" P is also presented.

"BA" aggregates are the result of simple "ballistic agglomeration", where projectiles approach on random, rectilinear trajectories, and "stick" if the trajectory brings them into contact with the cluster. BA aggregates are obtained if the projectile remains at the point of first contact. BA aggregates are quite porous. If the projectiles are equal-sized spheres, the porosity varies from P = 0.760 +/- 0.042 for N=16, increasing to P=0.884 +/- 0.006 for N=32768. (Figure 2 in Shen, Draine, and Johnson 2008 shows a plot of porosity P vs N)

"BAM1" aggregates are obtained if the "growth rule" is changed so that, after first contact, the newly-arrived spherule is allowed to "migrate" to make contact with a second sphere from the cluster. BAM1 aggregates are less porous than BA aggregates. N=16 clusters have P=0.579 +/- 0.036, with the porosity increasing to P=0.817 +/- 0.007 as N is increased to N=32768.

"BAM2" aggregates are obtained when the growth rule allows newly-arrived spherules to make, when possible, a second migration to produce contact with a third sphere (while remaining in contact with the first sphere that was contacted). BAM2 aggregates are less porous than BAM1 aggregates. N=16 clusters have P=0.401 +/- 0.026, with P increasing to 0.719 +/- 0.005 as N is increased to N=32768. Here we make available a "library" of BA, BAM1, and BAM2 clusters for N=8, 16, 32, 64, 128, 256, 512, 1024, and 2048. For each N value, we provide 16 independent random realizations. Each ascii file gives the x,y,z locations of the center of each sphere in the cluster. Important note: the clusters are NOT randomly-oriented. Instead, each cluster has been rotated to minimize the volume of a circumscribing rectangular "box" with axes parallel to the x,y,z axes. [This is done because one may wish to carry out calculations of light scattering and absorption using DDSCAT, which will be done more efficiently if the rectangular "computational volume" is minimized.]

BA clusters of spheres (monodisperse):

BAM1 clusters of spheres (monodisperse):

BAM2 clusters of spheres (monodisperse):

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