High Radiation Pressure on Interstellar Dust Computed by Light-Scattering Simulation on Fluffy Agglomerates of Magnesium-silicate Grains with Metallic-iron Inclusions

Abstract

Recent space missions have provided information on the physical and chemical properties of interstellar grains such as the ratio $\beta$ of radiation pressure to gravity acting on the grains in addition to the composition, structure, and size distribution of the grains. Numerical simulation on the trajectories of interstellar grains captured by Stardust and returned to Earth constrained the $\beta$ ratio for the Stardust samples of interstellar origin. However, recent accurate calculations of radiation pressure cross sections for model dust grains have given conflicting stories in the $\beta$ ratio of interstellar grains. The $\beta$ ratio for model dust grains of so-called "astronomical silicate" in the femto-kilogram range lies below unity, in conflict with $\beta \sim 1$ for the Stardust interstellar grains. Here, I tackle this conundrum by re-evaluating the $\beta$ ratio of interstellar grains on the assumption that the grains are aggregated particles grown by coagulation and composed of amorphous MgSiO$_{3}$ with the inclusion of metallic iron. My model is entirely consistent with the depletion and the correlation of major rock-forming elements in the Local Interstellar Cloud surrounding the Sun and the mineralogical identification of interstellar grains in the Stardust and Cassini missions. I find that my model dust particles fulfill the constraints on the $\beta$ ratio derived from not only the Stardust mission but also the Ulysses and Cassini missions. My results suggest that iron is not incorporated into silicates but exists as metal, contrary to the majority of interstellar dust models available to date.