5:15 PM - 7:15 PM
[PCG21-P02] Chemical Diversity of Circumstellar Dust Shells of Oxygen-rich AGB Stars
Introduction: Asymptotic giant branch (AGB) stars are an important source of cosmic dust [1]. Dust around AGB stars can be regarded as ingredients of the solar system because many presolar grains originated from AGB stars [2]. The difference in dust emission from AGB stars has been interpreted by the different combinations and fractions of various dust species [3]. It has been suggested that non-stoichiometric amorphous dust explains the observed spectrum showing a broad peak at 11-12 µm well and that increasing Fe content in oxides shifts their IR peak to longer wavelengths [4]. However, the KBr medium effects on their spectra cannot be ignored to compare them with the observation. Also, the 20 µm feature remains unexplained. Amorphous magnesiowüstite is suggested as the carrier of the 20 µm feature [5], but a reasonable explanation has not been made for why magnesiowüstite rather than silicates is preferentially formed in circumstellar environments. In this study, we determined the optical functions of five non-stoichiometric Ca-Al-Fe-Si oxides, including newly synthesized Al-Fe oxides containing Ca and Si, and applied them to a radiative transfer model to constrain the circumstellar dust composition.
Method: The condensation experiment was conducted with an induction thermal plasma (ITP) system [6]. The composition of the starting materials is Ca:Al:Fe:Si = 2:10:47:1. Then, we determined the optical functions of the product and 4 other ITP products (Al90Si10Ox, Ca0.75Al5.4SiOx, Ca1.7Al9.57Fe1.97SiOx, Ca2.3Al11.2Fe13SiOx) assuming the Lorentz Oscillator Model [7]. We modelled the IR spectra of dusty AGB shells using the radiative transfer model with DUSTY [8]. We assumed 10 dust species: amorphous olivine, amorphous pyroxene, amorphous melilite, 5 experimental oxides and silicates synthesized in the ITP system, amorphous magnesiowüstite, and crystalline spinel.
Results and Discussion: We obtained IR spectra of non-stoichiometric oxides in vacuum. The products of this study (Ca:Al:Fe:Si = 2:10:47:1) show a peak at ~15.5 µm. The 10–15 µm features of dust shells around O-rich AGB stars were better fitted by non-stoichiometric amorphous Ca-Al-Fe-Si oxides than stoichiometric amorphous silicates and oxides. However, our model did not reproduce a broad peak around 30 µm, which indicates the additional contribution of the low-temperature dust component. The strength of the 20 µm feature correlates with that of the 13 µm feature. This correlation indicates that the carrier of the 20 µm feature is refractory dust because the 13 µm feature has been explained by refractory oxides (spinel or corundum) [9]. Based on the relationship between dust composition and the optical thickness of the dust shell in the model fitting, the different C/O ratios in the atmospheres of AGB stars may be attributed to the chemical diversity of dust.
References: [1] Henning, T. (2010). Ann. Rev. A&A, 48, 21. [2] Zinner E. (2014). Treatise on Geo chemistry 2nd Edition 181.[3] Heras, A. M. & Hony, S. (2005). A&A, 439, 171. [4] Takigawa, A. et al. (2023), LPS LVI, #2136. [5] Henning, T. et al. (1995). A&A, 112, 143. [6] Kim, T. H. et al., 2021, A&A 656, A42. [7] Enomoto,H & Takigawa, A. (2024). LPI, #3036 [8] Ivezic, Z. et al. (1999). User Manual for DUSTY (http://www.pa.uky.edu/ moshe/dusty). [9] Takigawa, A. et al. (2015). ApJS, 218.
Method: The condensation experiment was conducted with an induction thermal plasma (ITP) system [6]. The composition of the starting materials is Ca:Al:Fe:Si = 2:10:47:1. Then, we determined the optical functions of the product and 4 other ITP products (Al90Si10Ox, Ca0.75Al5.4SiOx, Ca1.7Al9.57Fe1.97SiOx, Ca2.3Al11.2Fe13SiOx) assuming the Lorentz Oscillator Model [7]. We modelled the IR spectra of dusty AGB shells using the radiative transfer model with DUSTY [8]. We assumed 10 dust species: amorphous olivine, amorphous pyroxene, amorphous melilite, 5 experimental oxides and silicates synthesized in the ITP system, amorphous magnesiowüstite, and crystalline spinel.
Results and Discussion: We obtained IR spectra of non-stoichiometric oxides in vacuum. The products of this study (Ca:Al:Fe:Si = 2:10:47:1) show a peak at ~15.5 µm. The 10–15 µm features of dust shells around O-rich AGB stars were better fitted by non-stoichiometric amorphous Ca-Al-Fe-Si oxides than stoichiometric amorphous silicates and oxides. However, our model did not reproduce a broad peak around 30 µm, which indicates the additional contribution of the low-temperature dust component. The strength of the 20 µm feature correlates with that of the 13 µm feature. This correlation indicates that the carrier of the 20 µm feature is refractory dust because the 13 µm feature has been explained by refractory oxides (spinel or corundum) [9]. Based on the relationship between dust composition and the optical thickness of the dust shell in the model fitting, the different C/O ratios in the atmospheres of AGB stars may be attributed to the chemical diversity of dust.
References: [1] Henning, T. (2010). Ann. Rev. A&A, 48, 21. [2] Zinner E. (2014). Treatise on Geo chemistry 2nd Edition 181.[3] Heras, A. M. & Hony, S. (2005). A&A, 439, 171. [4] Takigawa, A. et al. (2023), LPS LVI, #2136. [5] Henning, T. et al. (1995). A&A, 112, 143. [6] Kim, T. H. et al., 2021, A&A 656, A42. [7] Enomoto,H & Takigawa, A. (2024). LPI, #3036 [8] Ivezic, Z. et al. (1999). User Manual for DUSTY (http://www.pa.uky.edu/ moshe/dusty). [9] Takigawa, A. et al. (2015). ApJS, 218.