Dust growth by accretion of molecules in supersonic interstellar turbulence

TL;DR

This study demonstrates that turbulence in cold molecular clouds significantly accelerates dust grain growth through accretion, broadening the grain-size distribution and evolving it towards a lognormal shape, with effects amplified in hypersonic turbulence.

Contribution

It provides the first detailed numerical simulation showing how turbulence enhances dust growth and shapes the grain-size distribution in interstellar environments.

Findings

Growth rate proportional to gas-density variance

Grain-size distribution tends to a lognormal shape

Decoupling in hypersonic turbulence accelerates growth

Abstract

We show that the growth rate of dust grains in cold molecular clouds is enhanced by the high degree of compressibility of a turbulent, dilute gas. By means of high resolution (10243) numerical simulations, we confirm the theory that the spatial mean growth rate is proportional to the gas-density variance. This also results in broadening of the grain-size distribution (GSD) due to turbulence-induced variation of the grain-growth rate. We show, for the first time in a detailed numerical simulation of hydrodynamic turbulence, that the GSD evolves towards a shape which is a reflection of the gas-density distribution, regardless of the initial distribution. That is, in case of isothermal, rotationally forced turbulence, the GSD tends to be a lognormal distribution. We also show that in hypersonic turbulence, decoupling of gas and dust becomes important and that this leads to an even further accelerated grain growth.