Iron and silicate dust growth in the Galactic interstellar medium: clues from element depletions

TL;DR

This study models dust growth and destruction in the interstellar medium, explaining observed element depletions and the distribution of iron and silicate grains, highlighting the importance of in-situ growth and grain composition.

Contribution

It extends previous dust evolution models to include iron, explaining its depletion patterns and distribution in different interstellar medium phases.

Findings

Iron is mostly locked in silicate inclusions, protecting it from destruction.

Depletion patterns are explained by dust growth in cold ISM and destruction by supernova shocks.

Iron nanoparticles account for the remaining iron depletion at high densities.

Abstract

The interstellar abundances of refractory elements indicate a substantial depletion from the gas phase, that increases with gas density. Our recent model of dust evolution, based on hydrodynamic simulations of the lifecycle of giant molecular clouds (GMCs) proves that the observed trend for [Si$_{gas}$/H] is driven by a combination of dust growth by accretion in the cold diffuse interstellar medium (ISM) and efficient destruction by supernova (SN) shocks (Zhukovska et al. 2016). With an analytic model of dust evolution, we demonstrate that even with optimistic assumptions for the dust input from stars and without destruction of grains by SNe it is impossible to match the observed [Si$_{gas}$/H]$-n_H$ relation without growth in the ISM. We extend the framework developed in our previous work for silicates to include the evolution of iron grains and address a long-standing conundrum: ``Where is the interstellar iron?'. Much higher depletion of Fe in the warm neutral medium compared to Si is reproduced by the models, in which a large fraction of interstellar iron (70%) is locked as inclusions in silicate grains, where it is protected from sputtering by SN shocks. The slope of the observed [Fe$_{gas}$/H]$-n_H$ relation is reproduced if the remaining depleted iron resides in a population of metallic iron nanoparticles with sizes in the range of 1-10nm. Enhanced collision rates due to the Coulomb focusing are important for both silicate and iron dust models to match the observed slopes of the relations between depletion and density and the magnitudes of depletion at high density.