5:15 PM - 7:15 PM
[PCG21-P03] Condensation experiments of Fe-bearing silicates using the MBE system
The lifetime of silicate grains in the interstellar medium (ISM) is estimated to be shorter than the typical stardust injection timescale [1]. On the other hand, the gas phase in the ISM is depleted in refractory elements, e.g., Mg, Si, and Fe by more than 90% [e.g., 2,3], indicating that these elements were present in refractory dust. Dust condensation in the dense ISM has been proposed to explain this discrepancy [4]. The mid-infrared spectrum of interstellar silicate has broad features at 10 and 18 µm, and the profile fits with the mixture of laboratory-synthesized amorphous silicates with Fe-rich olivine and pyroxene compositions [6-8]. We have developed a molecular beam epitaxy (MBE) apparatus to perform condensation of silicate at low temperatures [5]. In this study, we added another heating system to the MBE apparatus to introduce Fe gaseous molecules in addition to the two electron-beam heating systems to vaporize SiO2 and MgO. We controlled the fluxes for vapors from SiO2, MgO, and Fe metal independently and conducted condensation experiments in the Mg-Fe-Si-O system at room temperature, changing the Mg# and Me2+/Si ratios. KBr pellets (φ10 mm) were used as substrates for condensation. The infrared transmittance spectrum of the sample was measured by FT-IR (Thermo Nicolet iS5). The chemical compositions of the condensates were determined by SEM-EDS. TEM sections of some samples were prepared with a focused ion beam (FEI Versa3D Dualbeam) and observed by STEM-EDS (JEOL JEM-2800).
The FT-IR spectra of the condensate show broad peaks at 10 and 20 µm, indicating the formation of amorphous Mg-Fe silicates [9]. With increasing the Fe content, the 10 peak slightly shifted to shorter wavelengths, the 20 µm band peak shifted to longer wavelengths. The 10 µm dependence on Mg# is consistent with those of Mg-Fe silicate glass [9].
With increasing Me2+/Si ratio, the 10 µm peak shifted to longer, and the 20 μm peaks shifted to shorter wavelengths. The peak intensity ratio of the 20 µm feature relative to the 10 µm feature decreased. Amorphous silicates of Me2+/Si > 2 show only a slight difference in the IR spectra compared with that of amorphous olivine, which suggests that it is challenging to distinguish them by astronomical observations. The STEM-DF image and STEM-EDS map of the cross-section of the condensate with Fo67 composition showed a homogeneous distribution of Fe/Mg/Si. High-resolution TEM images and electron diffraction patterns showed that amorphous Mg-Fe silicates were formed on the KBr pellet. These observations suggest that Fe is dissolved in the silicate as Fe2+ or Fe3+ rather than forming metallic Fe. We will also show TEM results on the sample with Me2+/Si > 2 in the presentation.
References: [1] Jones A. P. et al. (1994) ApJ 433, 797. [2] Mathis J. S. (1990) Annu. Rev. Astron. Astrophys. 28, 37. [3] Savage B. D. and Sembach K. R. (1996) Annu. Rev. Astron. Astrophys. 34, 279. [4] Draine, B. T. (2009) ASPC, 414, 453. [5] Takigawa A. & Ueda H. (2024) MAPS #6304. [6] Kemper F. et al. (2004) ApJ 609, 826. [7] Kemper F. et al. (2005) ApJ 633, 534. [8] Chair J. E. and Tielens A. G. G. M. (2006) ApJ 637, 774. [9] Dorschnar J. et al. (1995) A&A, 300, 503.
frared spectrum.
The FT-IR spectra of the condensate show broad peaks at 10 and 20 µm, indicating the formation of amorphous Mg-Fe silicates [9]. With increasing the Fe content, the 10 peak slightly shifted to shorter wavelengths, the 20 µm band peak shifted to longer wavelengths. The 10 µm dependence on Mg# is consistent with those of Mg-Fe silicate glass [9].
With increasing Me2+/Si ratio, the 10 µm peak shifted to longer, and the 20 μm peaks shifted to shorter wavelengths. The peak intensity ratio of the 20 µm feature relative to the 10 µm feature decreased. Amorphous silicates of Me2+/Si > 2 show only a slight difference in the IR spectra compared with that of amorphous olivine, which suggests that it is challenging to distinguish them by astronomical observations. The STEM-DF image and STEM-EDS map of the cross-section of the condensate with Fo67 composition showed a homogeneous distribution of Fe/Mg/Si. High-resolution TEM images and electron diffraction patterns showed that amorphous Mg-Fe silicates were formed on the KBr pellet. These observations suggest that Fe is dissolved in the silicate as Fe2+ or Fe3+ rather than forming metallic Fe. We will also show TEM results on the sample with Me2+/Si > 2 in the presentation.
References: [1] Jones A. P. et al. (1994) ApJ 433, 797. [2] Mathis J. S. (1990) Annu. Rev. Astron. Astrophys. 28, 37. [3] Savage B. D. and Sembach K. R. (1996) Annu. Rev. Astron. Astrophys. 34, 279. [4] Draine, B. T. (2009) ASPC, 414, 453. [5] Takigawa A. & Ueda H. (2024) MAPS #6304. [6] Kemper F. et al. (2004) ApJ 609, 826. [7] Kemper F. et al. (2005) ApJ 633, 534. [8] Chair J. E. and Tielens A. G. G. M. (2006) ApJ 637, 774. [9] Dorschnar J. et al. (1995) A&A, 300, 503.
frared spectrum.