Effect of non-spherical projectiles on the structure of porous dust aggregates formed by coagulation

Abstract

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.