[P-CG22] Origin and evolution of materials in space

[PCG22-04] Constraint on the Chemical Composition of Circumstellar Dust from Optical Properties of Synthesized Amorphous Silicates

*Hanako Enomoto¹, Aki Takigawa¹ (1. Department of Earth and Planetary Science, The University of Tokyo)

Keywords:circumstellar dust, ISM, experiment, spectroscopy

Introduction: Asymptotic giant branch (AGB) stars are a major source of interstellar dust, and a major origin of presolar grains [1]. Mid-infrared spectroscopic observations of oxygen-rich AGB stars show absorption features at ~10 and ~18 µm originating from amorphous silicate and their exact peak positions and strength ratios vary among stars. Synthetic optical constants collectively called “astronomical silicate” [3, 4] have been widely used in the interpretation of observations of circumstellar and interstellar dust. These optical properties, however, do not represent real solid materials because they are arbitrarily synthesized from the combination of the observation and laboratory data. Thus, the nature of circumstellar dust, such as its chemical composition and structure, cannot be discussed using astronomical silicate. Although various dust analogs have been produced in laboratories including glassy silicates and condensed nanoparticles so-called smoke [e.g., 5, 6], their optical constants have not yet replaced astronomical silicate. This is because the chemical compositions of quenched melts are limited, and the reflectance of condensed particles is difficult to measure due to scattering by grains and the optical constants were not determined.

Methods: In this study, we produced dust analog nanoparticles with various chemical compositions and determined their optical constants by measuring reflectance and absorption spectra. We conducted gaseous condensation experiments in the system of Na-Al-Ca-Mg-Fe-Ni-Si-O using an induction thermal plasma (ITP) system (JEOL TP-40020NPS, [7]). The products were analyzed by XRD (Rigaku RINT-2100), EPMA (JEOL JXA-8530F), and TEM (JEOL JEM-2800). IR transmittance spectra of the condensates were measured with KBr pellets (JASCO FT/IR-4200), and the reflectance spectra were obtained (Thermo NICOLET6700) with pressed pellets of the particles.

Results and discussion: Condensed grains were mainly spherical amorphous nanoparticles of 10–100 nm in diameter. The bulk chemical compositions of the products slightly changed from the starting materials due to the incomplete vaporization of the starting materials. The transmittance and reflectance spectra were fitted with the Lorentz oscillator model and the complex refractive index (optical constants) was derived. Absorption spectra without KBr medium effect were calculated with determined optical constants. The medium effect gives peak shifts of ~0.5 µm on MIR spectra, which can be significant compared to the spectral change with chemical composition. The spectral change of experimental products indicates 10 and 18 µm peak positions systematically depend on the Al and Ca content relative to the CI chondritic composition. We compared the spectra of experimental products corrected the KBr effect, astronomical silicate, and observed dust emissions. the optical constants of dust analogs can be more quantitatively determined by the combination of absorption and reflectance spectra We concluded that the optical constants of dust analogs can be more quantitatively determined by the combination of absorption and reflectance spectra and that circumstellar silicate dust must not be pure Mg-Fe-silicates but could be rich in Al and Ca in order to explain the spectra of observed circumstellar dust, which has been interpreted by astronomical silicate.

References: [1] A. N. Nguyen et al. (2016) ApJ, 818, 51. [2] F. Kemper, W. J. et al., 2005, ApJ 633, 534. [3] B. T. Draine & H. M. Lee, 1984, ApJ 285, 89. [4] V. Ossenkopf et al., 1992, A&A 261, 567. [5] J. Dorschner, et al., 1995, A&A 300, 503. [6] Rietmeijer, F. J., et al., 1986, Icarus 66(2), 211. [7] T.H. Kim, et al., 2021, A&A 656, A42.