Dust dynamics in RAMSES -- I. Methods and turbulent acceleration

TL;DR

This paper introduces novel MHD-PIC methods in RAMSES to simulate dust grain acceleration, revealing that grains can reach shattering velocities in turbulent interstellar conditions, with implications for dust evolution.

Contribution

The paper develops and validates new MHD-PIC techniques in RAMSES for simulating dust dynamics, enabling detailed study of grain acceleration mechanisms in turbulent media.

Findings

Grains can be accelerated beyond shattering velocities in cold neutral medium conditions.

Charged grains exhibit extended velocity tails due to Lorentz force effects.

Gas-grain coupling influences the likelihood of grain shattering.

Abstract

Supernova ejecta and stellar winds are believed to produce interstellar dust grains with relatively large sizes. Smaller grains can be produced via the shattering of large grains that have been stochastically accelerated. To understand this stochastic acceleration, we have implemented novel magnetohydrodynamic(MHD)-particle-in-cell(PIC) methods into the astrophysical fluid code RAMSES. We treat dust grains as a set of massive ``superparticles'' that experience aerodynamic drag and Lorentz force. We subject our code to a range of numerical tests designed to validate our method in different physical conditions, as well as to illustrate possible mechanisms by which grains can be accelerated. As a final test as well as a foundation for future work, we present the results of decaying dusty MHD turbulence simulations with grain parameters chosen to resemble 1-2 $\mu$m grains in typical cold neutral medium conditions. We find that in these conditions, these grains can be effectively accelerated to well beyond their shattering velocities. This is true for both electrically charged and neutral grains. While the peak of the gas-grain relative drift velocity distribution is higher for neutral grains, the drift velocity distribution of charged grains exhibits an extended exponential tail out to much greater velocities. Even so, the shapes of the distributions are such that the extra gas-grain coupling provided by the Lorentz force offers grains relative protection from shattering. We also discuss the connection between our simulations and the relatively pristine ~$\mu$m sized presolar grains that do not appear to have undergone significant wear in their lifetimes.