Optical properties of elongated conducting grains

TL;DR

This paper calculates the optical absorption cross sections of elongated metallic dust grains, specifically iron needles, across a broad wavelength range using advanced computational methods, to better understand their astrophysical implications.

Contribution

It provides rigorous absorption cross section calculations for iron needles with various aspect ratios, challenging previous approximation methods like antenna theory and Rayleigh approximation.

Findings

Antenna theory and Rayleigh approximation are invalid for metallic needles.

Calculated absorption cross sections support earlier findings about needle optics.

Results aid in understanding dust's role in astrophysical phenomena.

Abstract

Extremely elongated, conducting dust particles (also known as metallic "needles" or "whiskers") are seen in carbonaceous chondrites and in samples brought back from the Itokawa asteroid. Their formation in protostellar nebulae and subsequent injection into the interstellar medium have been demonstrated, both experimentally and theoretically. Metallic needles have been suggested to explain a wide variety of astrophysical phenomena, ranging from the mid-infrared interstellar extinction at ~3--8 micron to the thermalization of starlight to generate the cosmic microwave background. To validate (or invalidate) these suggestions, an accurate knowledge of the optics (e.g., the amplitude and the wavelength dependence of the absorption cross sections) of metallic needles is crucial. Here we calculate the absorption cross sections of iron needles of various aspect ratios over a wide wavelength range, by exploiting the discrete dipole approximation, the most powerful technique for rigorously calculating the optics of irregular or nonspherical grains. Our calculations support the earlier findings that the antenna theory and the Rayleigh approximation, which are often taken to approximate the optical properties of metallic needles are indeed inapplicable.