The efficiency of grain growth in the diffuse interstellar medium

TL;DR

This paper investigates grain growth mechanisms in the diffuse interstellar medium, showing that Coulomb attraction leads to size growth until grains become positively charged, and suggests alternative explanations for observed elemental depletions.

Contribution

It demonstrates that charge effects limit grain growth in the diffuse ISM and proposes that initial depletions and inefficient dust destruction better explain observations.

Findings

Coulomb attraction causes small grains to grow until positively charged.

Constant grain size assumptions overestimate depletions.

Higher initial depletions and low destruction rates explain observations.

Abstract

Grain growth by accretion of gas-phase metals is a common assumption in models of dust evolution, but in dense gas, where the timescale is short enough for accretion to be effective, material is accreted in the form of ice mantles rather than adding to the refractory grain mass. It has been suggested that negatively-charged small grains in the diffuse interstellar medium (ISM) can accrete efficiently due to the Coulomb attraction of positively-charged ions, avoiding this issue. We show that this inevitably results in the growth of the small-grain radii until they become positively charged, at which point further growth is effectively halted. The resulting gas-phase depletions under diffuse ISM conditions are significantly overestimated when a constant grain size distribution is assumed. While observed depletions can be reproduced by changing the initial size distribution or assuming highly efficient grain shattering, both options result in unrealistic levels of far-ultraviolet extinction. We suggest that the observed elemental depletions in the diffuse ISM are better explained by higher initial depletions, combined with inefficient dust destruction by supernovae at moderate ($n_{\rm H} \sim 30 {\rm \, cm^{-3}}$) densities, rather than by higher accretion efficiences.