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

TL;DR

This study uses molecular dynamics simulations to determine collision outcomes of silicate dust grains, updating threshold velocities and size distributions relevant for interstellar dust evolution models.

Contribution

It provides new, empirically derived threshold velocities and size distribution data for silicate grain collisions, correcting previous models and assumptions.

Findings

Grain shattering thresholds are around 6 km/s, higher than previous estimates.

Size distributions of shattered grains differ from earlier power-law predictions.

Updated velocity thresholds suggest dust grains are more resistant to shattering.

Abstract

(abridged) We aim to predict the most important parameters for grain-grain collision outcomes for models of interstellar grain population evolution on astrophysical scales: the threshold velocity above which colliding grains shatter, the threshold for vaporization, and 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 $a \in [5,50]~\AA$ 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 $\sim$6 km/s for both amorphous SiO$_2$ and astrodust material, which is a factor of $\sim$2 higher than the canonical value for silicates of 2.7 km/s from Jones et al. (1996). This discrepancy is mostly alleviated by correcting an error in the expression for these velocity thresholds derived in Tielens et al. (1994). We find that the size distributions of shattered products are generally not consistent with the power law distributions predicted by this previous model. We also find that their expression fails to predict the fraction of shattered or vaporized material observed in our numerical simulations. The model of Hirashita \& Kobayashi (2013) for the same quantities similarly fails to match the simulations. We provide updated shattering velocity thresholds for standard candidate grain materials to the astrophysics community. Broadly, our updated threshold velocities that astrophysical dust grains may be more robust to shattering in the interstellar medium than previously assumed.