Hidden metallic iron in amorphous silicate dust? Insights from condensation experiments and mid-infrared spectroscopy

TL;DR

This study investigates the presence of metallic iron in amorphous silicate dust by producing and analyzing nanoparticles with and without metallic cores, providing insights into dust composition in space.

Contribution

It introduces experimental methods to produce amorphous silicate nanoparticles with metallic cores and measures their optical properties, advancing understanding of dust composition.

Findings

Produced amorphous silicate nanoparticles with metallic cores

Measured optical constants and spectral features of the dust analogues

Suggests metallic core-containing silicates are common in space dust

Abstract

Amorphous silicate dust is a major component in the interstellar and circumstellar dust formed in the outflow of asymptotic giant branch (AGB) stars. Although iron depletion is observed in the interstellar medium (ISM), the exact form and fraction of iron in solid remains under debate. In particular, it is unclear whether amorphous silicate dust around AGB stars contains metallic iron. We aimed to provide optical constants of amorphous silicate nanoparticles and examine the effects of metallic iron on their spectral features to better constrain the dust properties by producing amorphous silicate nanoparticles with and without metallic cores. We performed condensation experiments using an induction thermal plasma system to produce dust analogues of the CI chondritic composition in the Mg-Ca-Na-Al-Si-Fe-Ni-O and Mg-Ca-Na-Al-Si-O systems. We measured the absorbance and reflectance of the samples, observed the structure of the products, and determined the optical constants. Two types of amorphous silicate nanoparticles (10-200 nm in diameter) with nearly CI chondritic composition were produced: one contained kamacite (Fe: Ni=0.9: 0.1) cores with a diameter ratio ranging 0-0.87 (average ~0.50), and the other was iron-free homogeneous amorphous silicate. The amorphous silicates of the CI chondritic composition with various sized metallic cores may be prevalent in circumstellar and interstellar dust.