AI 中文总结
该研究通过软球离散元模拟，探究非球形弹丸对凝并形成的多孔尘埃聚集体结构的影响，发现聚集体结构参数的相关依赖关系，指出尘埃凝并后需额外处理才能匹配观测结果。
AI 中文摘要
宇宙尘埃在宇宙中无处不在，但单个尘埃颗粒的结构与几何形态仍知之甚少。数值模拟预测存在非球形、蓬松甚至分形的颗粒结构，且恒星光的线偏振观测也支持这一点，但要对这类关键颗粒特征开展超出定性的研究已被证实颇具挑战。我们采用软球离散元法开展尘埃凝并模拟，使用不同尺寸的非球形弹丸进行连续碰撞，在三种生长条件下研究所得聚集体的内部结构与几何形态：恒定最终聚集体尺寸、恒定弹丸数量、恒定弹丸尺寸。多数情况下我们允许弹丸发生内部重排，但也测试了撞击后强制弹丸结构保持恒定的影响。研究发现，聚集体的孔隙率与分形维数同时取决于弹丸的尺寸和数量，而其不对称性与拉伸参数更多取决于弹丸数量而非尺寸；总体而言，颗粒孔隙率随弹丸数量及单个弹丸尺寸的增加而增大。与星际偏振约束的对比表明，我们模拟的足够大的聚集体中，没有任何结构能够重现星际介质中观测到的恒星光偏振。我们得出结论：宇宙尘埃在凝并后会经历额外的处理过程，以形成与观测相符的结构。
英文摘要
Cosmic dust is ubiquitous in the universe, yet the structure and geometry of individual grains remain poorly understood. The existence of non-spherical, fluffy, and even fractal grain structures is predicted by numerical simulation and supported by observations of linear polarization of starlight. However, it has proven challenging to go beyond a qualitative investigation of such crucial grain characteristics. We present soft-sphere discrete element simulations of dust coagulation with sequential collisions using non-spherical projectiles of various sizes. We study the internal structure and geometry of the resulting aggregates under three growth conditions: a constant final aggregate size, a constant number of projectiles, and a constant projectile size. In most cases we allow for projectile internal restructuring, but we also test the effect of enforcing a constant projectile structure after impact. We find that aggregates' porosity and fractal dimension depend both on the size and number of projectiles, and that their asymmetry and stretch parameters depend more on the number of projectiles than on projectile size. Overall, the grain porosity increases with both the number of projectiles and the size of the individual projectiles. Comparison with constraints from interstellar polarization indicates that none of our sufficiently large aggregates have structures capable of reproducing the observed polarization of starlight in the interstellar medium. We conclude that cosmic dust undergoes additional processing after coagulation to acquire structures consistent with observations.
Comments14 pages and 9 figures
