On the Absorption Properties of Metallic Needles

TL;DR

This paper critiques the use of the antenna approximation for metallic needle absorption cross sections in astrophysics, emphasizing the need for rigorous calculations and experimental validation to better understand their properties.

Contribution

It highlights the inaccuracy of the antenna approximation and advocates for precise theoretical and experimental studies of metallic needle absorption properties.

Findings

Antenna approximation violates Kramers-Kronig relations.

Calls for discrete dipole approximation calculations.

Recommends experimental measurements of absorption cross sections.

Abstract

Needle-like metallic particles have been suggested to explain a wide variety of astrophysical phenomena, ranging from the mid-infrared interstellar extinction to the thermalization of starlight to generate the cosmic microwave background. These suggestions rely on the amplitude and the wavelength dependence of the absorption cross sections of metallic needles. On the absence of an exact solution to the absorption properties of metallic needles, their absorption cross sections are often derived from the antenna approximation. However, it is shown here that the antenna approximation is not an appropriate representation since it violates the Kramers-Kronig relation. Stimulated by the recent discovery of iron whiskers in asteroid Itokawa and graphite whiskers in carbonaceous chondrites, we call for rigorous calculations of the absorption cross sections of metallic needle-like particles, presumably with the discrete dipole approximation. We also call for experimental studies of the formation and growth mechanisms of metallic needle-like particles as well as experimental measurements of the absorption cross sections of metallic needles of various aspect ratios over a wide wavelength range to bound theoretical calculations.