Do twin spectral peaks of olivine particles in the thermal infrared diagnose their sizes and porosities?

TL;DR

This paper reexamines how twin spectral peaks in olivine's thermal infrared spectra can diagnose particle size and porosity, emphasizing the physics behind small-particle effects and the impact of porosity on spectral features.

Contribution

It provides a theoretical and numerical analysis clarifying the physics of spectral peaks, reaffirming their use as diagnostics for olivine grain size and porosity in astronomical dust.

Findings

Surface roughness effects are negligible for spectral peaks.

Porosity significantly affects peak strength and wavelength.

Twin peaks are intrinsic indicators of small, porous olivine grains.

Abstract

A well-established constraint on the size of non-porous olivine grains or the porosity of aggregates consisting of small olivine grains from prominent narrow peaks in thermal infrared spectra characteristic of crystalline silicates is reexamined. To thoroughly investigate thermal infrared peaks, we make theoretical argument for the absorption and scattering of light by non-porous, non-spherical olivine particles, which is followed by numerical verification. Our study provides perfectly rational explanations of the physics behind the small-particle effect of emission peaks in the framework of classical electrodynamics and convincing evidence of small-particle's emission peaks in the literature. While resonant absorption excited by surface roughness on the order of submicrometer scales can be identified even for non-porous olivine particles with a radius of $10~{\rm \mu m}$, it makes only a negligible contribution to thermal infrared spectra of the particles. In contrast, the porosity of non-spherical particles has a significant impact on the strength and wavelength of the peaks, while the resonant absorption excited by an ensemble of small grains takes place at a wavelength different than one expects for surface roughness. We finally reaffirm that twin peaks of olivine in thermal infrared spectra of dust particles in astronomical environments are the intrinsic diagnostic characters of submicrometer-sized small grains and their aggregate particles in fluffy and porous configurations.