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

TL;DR

This study investigates whether metallic iron can be embedded within amorphous silicate dust by synthesizing CI-chondritic amorphous silicate nanoparticles with and without Fe–Ni cores using an induction thermal plasma, then deriving their optical constants via Lorentz oscillator modeling from combined absorbance and reflectance data. The results show predominantly amorphous grains with occasional kamacite cores (Fe–Ni) and offer optical constants anchored by $oldsymbol{ extepsilon_ extinfty}$. Importantly, MIR spectral features around ~10 and ~18 μm are largely unchanged by metallic cores due to a dominance of small grains and core-size distributions, implying metallic iron in circumstellar dust could be spectrally hidden in the MIR. The work suggests that GEMS-like iron-bearing inclusions may be present in dust around AGB stars, and recommends using radiative transfer modeling with these optical constants to better constrain the presence and roles of metallic cores in circumstellar environments.

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.

Figures (15)

• Figure 1: XRD patterns of the starting material of CI-1 (upper panel) and experimental products (CI-1, CI-2; bottom panel). Peaks indicated by open symbols are derived from the reagents composing the starting material. Filled symbols show the peaks of new phases formed through the condensation.
• Figure 2: TEM bright field images of experimental products in CI-1 (a) and CI-2 (b), and STEM-EDS maps of experimental products in CI-1(c) and CI-2 (d).
• Figure 3: Histogram of the core size ratios, $r_{\rm{c}}/r$, of particles condensed in CI-1.
• Figure 4: FT-IR absorbance spectra of CI-1 (dashed line) and CI-2 (solid line).
• Figure 5: Secondary electron images of the surface of a thin (a) and a thick (b) Au-coated pellet of CI-2 taken with a $0.4 \mathrm{\, nA}$ electric beam at $5 \mathrm{\, kV}$ (JEOL; JSM-7000F).