11:45 AM - 12:00 PM
[PCG21-11] Experimental Constraints on Amorphous Silicate Dust: Investigating the Role of Metallic Iron Cores and Chemical Compositions in Infrared Spectra
Keywords:circumstellar dust, amorphous silicate, AGB stars, infrared spectroscopy, optical constants, experiments
Introduction: Amorphous silicate dust, a major component of circumstellar and interstellar refractory dust, is primarily formed in the outflow of asymptotic giant branch (AGB) stars [1]. Infrared spectra of AGB stars show various amorphous silicate features, implying that dust components can differ among stars. Circumstellar and interstellar dust have been interpreted with synthetic optical constants known as "astronomical silicate" [2, 3]. However, these optical constants were arbitrarily synthesized by combining observational and laboratory data and, consequently, cannot be used to infer the chemical composition of dust. To accurately constrain dust properties, optical constants of amorphous silicates across a wide range of chemical compositions are needed. While numerous laboratory studies have produced dust analogs [e.g., 4, 5, 6], their chemical compositions are mainly stoichiometric and insufficient to explain the diversity in observed dust spectra. In addition, these experimental studies suppose that amorphous silicate grains exist as pure oxides without considering the presence of metallic inclusions. The chemical state (oxides or metal) and spatial distribution (within amorphous silicate grains or as independent particles) of iron are key factors influencing the catalytic functions of dust. It is known that some glass with embedded metal and sulfides (GEMS) grains exhibit infrared spectra similar to those of circumstellar and interstellar dust [7]. To better constrain the chemical composition of amorphous silicate dust, the spectral effects of metallic iron cores must be experimentally investigated.
Methods: Condensation experiments in the system including Na, Al, Ca, Mg, Fe, Ni, Si, and O were carried out using the induction thermal plasma (ITP) system (JEOL TP-40020NPS, [8]) and produced dust analogs with Fe, Ni-free CI chondritic composition, metallic Fe, Ni-containing CI chondritic composition, systematically changing Al/Si, Ca/Mg ratios from CI chondritic composition. The products were analyzed by XRD (Rigaku RINT-2100), EPMA (JEOL JXA-8530F), Raman spectroscopy (BX-51, Olympus; 500is-sm, Bruker), and TEM (JEOL JEM-2800). Besides infrared transmittance measurements (JASCO FT/IR-4200), reflectance spectra of the condensates were measured (Thermo NICOLET6700) using pressed pellets of the particles with/without an Au coating. We determined the optical constants of products from transmittance and reflectance spectra, assuming the Lorentz oscillator model.
Results and discussion: The products were amorphous silicate nanoparticles (10-200 nm in diameter). In the system with Fe-Ni, silicate grains contained kamacite (Fe0.9Ni0.1) particles. The size proportion of metallic cores ranged from 0 to 0,87, and the average was 0.50. The spectral peak positions of product with Fe cores did not differ from product without Fe cores despite its Fe abundance (Fe/Si~0.83), nearly equal to the Solar abundance (Fe/Si~0.85)[9]. It is indicated that infrared spectra are dominated by the size distribution and number density of metallic cores. If the metallic irons condense in larger cores and most grains have small cores, the core may not be identified from observations. In the Mg-Ca-Al-Si-O system, the peaks shifted from 9.4 to 9.6 μm as Al/Si increased from 0.07 to 0.53, and from 9.4 to 9.7 μm and 17.7 to 19.1 μm as Ca/Mg increased from 0 to 1. The properties of observed dust emissions of some AGB stars were constrained with the calculated dust spectra using optical constants of our samples and assuming various grain sizes and dust temperatures, suggesting compositional diversity of AGB silicate dust.
References: [1] Henning, 2020, A&A 48, 21. [2] Draine & Lee, 1984, ApJ 285, 89. [3] Ossenkopf et al., 1992, A&A 261, 567. [4] Dorschner et al., 1995, A&A 300, 503. [5] Rietmeijer et al., 1986, Icarus 66(2), 211. [6] Mutschke et al., 1998, A&A 333, 188. [7] Bradley et al., 1999, Science 285, 5434. [8] Kim et al., 2021, A&A 656, A42. [9] Lodders et al., 2009, Solar System, 712-770.
Methods: Condensation experiments in the system including Na, Al, Ca, Mg, Fe, Ni, Si, and O were carried out using the induction thermal plasma (ITP) system (JEOL TP-40020NPS, [8]) and produced dust analogs with Fe, Ni-free CI chondritic composition, metallic Fe, Ni-containing CI chondritic composition, systematically changing Al/Si, Ca/Mg ratios from CI chondritic composition. The products were analyzed by XRD (Rigaku RINT-2100), EPMA (JEOL JXA-8530F), Raman spectroscopy (BX-51, Olympus; 500is-sm, Bruker), and TEM (JEOL JEM-2800). Besides infrared transmittance measurements (JASCO FT/IR-4200), reflectance spectra of the condensates were measured (Thermo NICOLET6700) using pressed pellets of the particles with/without an Au coating. We determined the optical constants of products from transmittance and reflectance spectra, assuming the Lorentz oscillator model.
Results and discussion: The products were amorphous silicate nanoparticles (10-200 nm in diameter). In the system with Fe-Ni, silicate grains contained kamacite (Fe0.9Ni0.1) particles. The size proportion of metallic cores ranged from 0 to 0,87, and the average was 0.50. The spectral peak positions of product with Fe cores did not differ from product without Fe cores despite its Fe abundance (Fe/Si~0.83), nearly equal to the Solar abundance (Fe/Si~0.85)[9]. It is indicated that infrared spectra are dominated by the size distribution and number density of metallic cores. If the metallic irons condense in larger cores and most grains have small cores, the core may not be identified from observations. In the Mg-Ca-Al-Si-O system, the peaks shifted from 9.4 to 9.6 μm as Al/Si increased from 0.07 to 0.53, and from 9.4 to 9.7 μm and 17.7 to 19.1 μm as Ca/Mg increased from 0 to 1. The properties of observed dust emissions of some AGB stars were constrained with the calculated dust spectra using optical constants of our samples and assuming various grain sizes and dust temperatures, suggesting compositional diversity of AGB silicate dust.
References: [1] Henning, 2020, A&A 48, 21. [2] Draine & Lee, 1984, ApJ 285, 89. [3] Ossenkopf et al., 1992, A&A 261, 567. [4] Dorschner et al., 1995, A&A 300, 503. [5] Rietmeijer et al., 1986, Icarus 66(2), 211. [6] Mutschke et al., 1998, A&A 333, 188. [7] Bradley et al., 1999, Science 285, 5434. [8] Kim et al., 2021, A&A 656, A42. [9] Lodders et al., 2009, Solar System, 712-770.