Thermodynamics and Charging of Interstellar Iron Nanoparticles
  Interstellar iron in the form of metallic iron nanoparticles may constitute a
component of the interstellar dust. We compute the stability of iron
nanoparticles to sublimation in the interstellar radiation field, finding that
iron clusters can persist down to a radius of $\simeq 4.5\,$\AA, and perhaps
smaller. We employ laboratory data on small iron clusters to compute the
photoelectric yields as a function of grain size and the resulting grain charge
distribution in various interstellar environments, finding that iron
nanoparticles can acquire negative charges particularly in regions with high
gas temperatures and ionization fractions. If $\gtrsim 10\%$ of the
interstellar iron is in the form of ultrasmall iron clusters, the photoelectric
heating rate from dust may be increased by up to tens of percent relative to
dust models with only carbonaceous and silicate grains.
