Optical properties of silicon carbide for astrophysical applications - I. New laboratory infrared reflectance spectra and optical constants

Aims. The SiC optical constants are fundamental inputs for radiative transfer (RT) models of astrophysical dust environments. However, previously published values contain errors and do not adequately represent the bulk physical properties of the cubic (beta) SiC polytype usually found around carbon stars. We provide new, uncompromised optical constants for beta- and alpha-SiC derived from single-crystal reflectance spectra and investigate quantitatively (i) whether there is any difference between alpha- and beta-SiC that can be seen in infrared (IR) spectra and optical functions and (ii) whether weak features from lambda similar to 12.5-13.0 mu m need to be fitted. Methods. We measured mid- and far-IR reflectance spectra for two samples of 3C (beta-) SiC and four samples of 6H (alpha-) SiC. For the latter group, we acquired polarized data (E perpendicular to c, E parallel to c orientations). We calculated the real and imaginary parts of the complex refractive index (n(lambda) + ik(lambda)) and the ideal absorption coefficients via classical dispersion fits to our reflectance spectra. Results. We find that beta-SiC and E perpendicular to c alpha-SiC have almost identical optical functions but that n(lambda) and k(lambda) for E parallel to c alpha-SiC are shifted to lower frequency. Peak positions determined for both 3C (beta-) and 6H (alpha-) SiC polytypes agree with Raman measurements and show that a systematic error of 4 cm(-1) exists in previously published IR analyses, attributable to inadequate resolution of older instruments for the steep, sharp modes of SiC. Weak modes are present for samples with impurities. Our calculated absorption coefficients are much higher than laboratory measurements. Whereas astrophysical dust grain sizes remain fairly unconstrained, SiC grains larger than about 1 mu m in diameter will be opaque at frequencies near the peak center. Conclusions. Previous optical constants for SiC do not reflect the true bulk properties, and they are only valid for a narrow grain size range. The new optical constants presented here will allow narrow constraints to be placed on the grain size and shape distribution that dominate in astrophysical environments.