The optical constants of crystalline silicate particles in mid- and far-infrared
Abstract
The optical constants from the mid to far infrared region are presented for the crystalline silicate particles with submicron size, i.e. olivine, forsterite, clinopyroxene and orthopyroxene particles which are considered as major constituents of interplanetary, interstellar and circumstellar dust. The olivine, clinoproxene and orthopyroxene are natural from Ichinomegata, Akita and have the following characteristics, olivine: (Mg_0.90Fe_0.10)_2SiO_2(Fo_90), rho = 3.36g/cm^3 clinopyroxene: Ca(Mg,Fe,Al)(Si,Al)_2O_6 Mg / (Mg+Fe) = 0.90 - 0.92, Al_2O_3 = 1.9 - 4.9 wt%, rho = 3.28 g/cm^3 orthopyroxene: (Mg,Fe,Al)(Si,Al)O_3 3.36g / cm^3 Mg / (Mg+Fe) = 0.89 - 0.90, Al_2O_3 = 3.3 - 4.0 wt%, rho = 3.32 g/cm^3 The forsterite (Mg_2SiO_4, [Mg / (Mg+Fe) = 1]) is synthesized by the CZ method (Takei 1976). The optical constants have been derived from the transmission spectra of the particles with use of Kramers-Kronig relation (Rouleau and Martin 1991). The transmissions from 5 microns to 30 microns and 25 microns to 100 microns were measured respectively for KBr pellets and polyethylene sheets containing the particles. Under Rayleigh approximation (lambda gg particle size) the imaginary part of the polarizability A per unit volume has a form of A = (epsilon/epsilon_h-1) / ((epsilon/epsilon_h-1)L+1) where epsilon: dielectric constant of particle, epsilon_h: dielectric constant of host medium and L: shape parameter of dust with assumption of homogeneous spheroid. The imaginary part of A is proportional to the transmission T as Aimg = lambda/{(2pi nhost)} rho/M ln (1/T). Imaginary part of A at infrared region are given from our measurements, and with applying Kramers-Kronig relation to the polarizability the real part of A can be derived, then the optical constants of particles are evaluated from equation of A. The silicate's strong absorption features around 10 microns and 11 microns were measured for the particles placed on the surface of KBr or glass plate (not embedded in KBr), in which case the ratios of the absorption depth of the features were found to have little effect of substrate medium, so the ratios obtained in this way should be determined from optical constant of particle itself. The ratios were successfully reproduced with our optical constants by adjusting the shape parameter L where the value L was confirmed to be consistent with the shape of the particles by inspecting their images under microscope.
- Publication:
-
IAU Symposium
- Pub Date:
- 1998
- Bibcode:
- 1998IAUS..191P.315S