An Experimental Study on the Structure of Cosmic Dust Aggregates and Their Alignment by Motion Relative to Gas

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Abstract
We experimentally studied the shape of dust grains grown in a cluster-cluster type of aggregation (CCA) and derived characteristic axial ratios to describe the nonsphericity. CCAs might be described by an axial ratio ρCCA = rg, max/rg, min ≈ 2.0 in the limit of large aggregates, where rg, min and rg, max describe the minimum and maximum radius of gyration, while small aggregates show a somewhat larger value in their mean axial ratio up to ρCCA ≈ 3.0 but rapidly decrease to the limit ρCCA ≈ 2.0. The axial ratios for large aggregates are in agreement with the general findings of different authors for axial ratios of interstellar dust grains that are generally described by rods or spheroids. Beyond this kind of agreement, our approach does not necessarily require a special shape for individual dust grains but rather offers a physical process to generate nonsphericity. Although the simple shapes might be sufficient for first-order applications and are easier to handle analytically, our results offer a firm ground of special axial ratios for rods or spheroids on a more physical basis apart from any ad hoc assumptions. We also find an alignment of the aggregates during sedimentation in a gas along the drift axis leading to an axial ratio of ρCCA, align = 1.21 ± 0.02 with respect to the drift axis and an axis perpendicular to this drift. This result is directly applicable to dust grains in protoplanetary disks and planetary atmospheres.