Silicate cosmic dust grain collisions in the interstellar medium: A molecular dynamics study

(abridged) We aim to predict the most important parameters for grain-grain collision outcomes in models of interstellar grain population evolution: the threshold velocities for shattering and vaporization, and the resulting distributions of grain sizes. We use molecular dynamics simulations which evolve the dynamics of each atom in a dust grain to explore the outcomes of collisions between silicate grains of radii a in [5,50] Å at velocities 0.1-20 km/s. We run simulations of grains with two materials: amorphous SiO 2 and an amorphous silicate of composition suggested by Draine & Hensley (2021). With these simulations, we quantify the collision velocity dependence of shattered and vaporized mass fractions, and the resulting size distributions of shattering products. We find grain shattering thresholds are about 6 km/s for both materials, a factor of about 2 higher than the canonical value for silicates of 2.7 km/s from Tielens et al. (1994) and Jones et al. (1996). The size distributions of shattered products are generally not consistent with the power law distributions predicted by Jones et al. (1996), but the physical reasons for these discrepancies remain uncertain. The Tielens et al. (1994) expressions fail to predict the fraction of shattered or vaporized material observed in our numerical simulations, over-predicting the shattered fraction and under-predicting the vaporized fraction, except for the most massive grains we simulate, where the predicted vaporized fraction agrees reasonably well. The model of Hirashita & Kobayashi (2013) similarly fails to match the simulations. We provide updated shattering velocity thresholds for nanoscale silicate grains, which suggest that the smallest astrophysical dust grains composed of silicate materials may be more robust to shattering in the interstellar medium than previously assumed.