Absorption Efficiencies of Forsterite. I. Discrete Dipole Approximation Explorations in Grain Shape and Size

Abstract

We compute the absorption efficiency (Q abs ) of forsterite using the discrete dipole approximation in order to identify and describe what characteristics of crystal grain shape and size are important to the shape, peak location, and relative strength of spectral features in the 8-40 μm wavelength range. Using the DDSCAT code, we compute Q abs for non-spherical polyhedral grain shapes with a eff = 0.1 μm. The shape characteristics identified are (1) elongation/reduction along one of three crystallographic axes; (2) asymmetry, such that all three crystallographic axes are of different lengths; and (3) the presence of crystalline faces that are not parallel to a specific crystallographic axis, e.g., non-rectangular prisms and (di)pyramids. Elongation/reduction dominates the locations and shapes of spectral features near 10, 11, 16, 23.5, 27, and 33.5 μm, while asymmetry and tips are secondary shape effects. Increasing grain sizes (0.1-1.0 μm) shifts the 10 and 11 μm features systematically toward longer wavelengths and relative to the 11 μm feature increases the strengths and slightly broadens the longer wavelength features. Seven spectral shape classes are established for crystallographic a-, b-, and c-axes and include columnar and platelet shapes plus non-elongated or equant grain shapes. The spectral shape classes and the effects of grain size have practical application in identifying or excluding columnar, platelet, or equant forsterite grain shapes in astrophysical environs. Identification of the shape characteristics of forsterite from 8 to 40 μm spectra provides a potential means to probe the temperatures at which forsterite formed.

Cometary dust: the diversity of primitive refractory grains

“…Preliminary computations of fayalite with rectangular-shaped crystals using the DDSCAT code, similar to the techniques in [155], suggest the Fe-rich olivine would need to be larger than approximately 5 μm for opacity effects to start to significantly affect the far-IR resonances. Fe-rich olivine in Stardust are 5–30 μm-size but these same composition crystals are not that large in giant CP IDPs.…”

“…The IR crystalline silicate resonances are best matched with forsterite and ortho-enstatite [ 32 , 150 ]. Laboratory spectra show that for increasing FeO contents the wavelengths of the spectral peaks shift to longer wavelengths [ 151 – 153 ] and these longer wavelengths do not match comet IR spectra [ 154 , 155 ].…”

“…6(e). Their San Carlos forsterite standard appears to have a longer wavelength around 860 cm −1 (11.63 μm) compared with laboratory absorption spectra of ground forsterite at 893–897 cm −1 (11.2–11.15 μm) [ 152 , 155 , 210 ]. IR absorption spectra of forsterite are sensitive to the crystal shapes, e.g.…”

“…IR absorption spectra of forsterite are sensitive to the crystal shapes, e.g. ball-grinding versus hand-grinding tends to produce more spherical-shapes for crystals and shifts some resonances to shorter wavelengths [ 210 ] and rectangular-prism-shaped forsterite crystals fit cometary IR spectra better than spheres or ellipsoids [ 155 ]. IR absorption spectra of forsterite also are sensitive to the temperature [ 153 ].…”

Laboratory measurement of optical constants of solid SiO and application to circumstellar dust

“…For spherical grains the calculated line profiles are located at somewhat shorter wavelengths A92, page 10 of 13 than the observed profile. They do not match observations well, but this is a well known problem if one assumes particles that are exactly spherical (see, e.g., Lindsay et al 2013, for a detailed discussion of this).…”