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

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.