Thermodynamics and Charging of Interstellar Iron Nanoparticles

TL;DR

This paper investigates the stability, charging, and heating effects of interstellar iron nanoparticles, showing they can survive in space and significantly influence interstellar heating if present in sufficient quantities.

Contribution

It provides new calculations on the stability, charge distribution, and heating impact of iron nanoparticles in interstellar environments, based on laboratory data and astrophysical modeling.

Findings

Iron nanoparticles can survive down to a radius of about 4.5 Å.

They can acquire negative charges in high-temperature, high-ionization regions.

Presence of iron nanoparticles can increase interstellar dust heating by up to tens of percent.

Abstract

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.