An experimental study on the structure of cosmic dust aggregates and theiralignment by motion relative to gas

We experimentally studied the shape of dust grains grown in a cluster-clust er type of aggregation (CCA) and derived characteristic axial ratios to des cribe the nonsphericity. CCAs might be described by an axial ratio rho(CCA) = gamma(g, max)/r(g), (min) approximate to 2.0 in the limit of large aggre gates, where r(g, min) and r(g), (max) describe the minimum and maximum rad ius of gyration, while small aggregates show a somewhat larger value in the ir mean axial ratio up to rho(CCA) approximate to 3.0 but rapidly decrease to the Limit rho(CCA) approximate to 2.0. The axial ratios for large aggreg ates are in agreement with the general findings of different authors for ax ial ratios of interstellar dust grains that are generally described by rods or spheroids. Beyond this kind of agreement, our approach does not necessa rily require a special shape for individual dust grains but rather offers a physical process to generate nonsphericity. Although the simple shapes mig ht be sufficient for first-order applications and are easier to handle anal ytically, 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 alo ng the drift axis leading to an axial ratio of rho(CCA, 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.