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 [Sigas/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
[Sigas/H]−nH 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
[Fegas/H]−nH 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.Comment: Accepted for publication in the ApJ, 15 pages, 9 figure
