Optical properties of silicon carbide for astrophysical applications

Abstract

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 (β) SiC polytype usually found around carbon stars. We provide new, uncompromised optical constants for β-and α-SiC derived from single-crystal reflectance spectra and investigate quantitatively (i) whether there is any difference between α-and β-SiC that can be seen in infrared (IR) spectra and optical functions and (ii) whether weak features from λ ∼ 12.5−13.0 µm need to be fitted. Methods. We measured mid-and far-IR reflectance spectra for two samples of 3C (β-)SiC and four samples of 6H (α-)SiC. For the latter group, we acquired polarized data (E⊥c, E c orientations). We calculated the real and imaginary parts of the complex refractive index (n(λ) + ik(λ)) and the ideal absorption coefficients via classical dispersion fits to our reflectance spectra. Results. We find that β-SiC and E⊥c α-SiC have almost identical optical functions but that n(λ) and k(λ) for E c α-SiC are shifted to lower frequency. Peak positions determined for both 3C (β-) and 6H (α-)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 µm 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.

Metamaterial Model of Tachyonic Dark Energy

“…Indeed, our calculations presented in Figure 2 indicate that around n ~ 0.3 a multilayer SiC-Si metamaterial does have pronounced closely spaced tachyonic and bradyonic frequency bands, while having relatively low losses. The calculated values of ε 1 and ε 2 are based on the measured optical properties of SiC reported in [22]. A somewhat similar result has been reported in [21] for a SiC-Si wire array structure, but unlike our multilayer design, fabrication of SiC-Si wire arrays may prove extremely difficult.…”

Modelling the atmosphere of the carbon-rich Mira RU Virginis

“…Fits for RU Vir with a lower SiC fraction were not as good. The optical constants for the opacities of silicon carbide are taken from Pitman et al (2008) and amorphous carbon from Rouleau & Martin (1991). Since the standard grain size distribution of DUSTY produced SiC features that were much too peaked, we used the distribution of hollow spheres (DHS, Groenewegen 2012, and references therein) with a mean grain size of the dust mixture of 0.2 µm.…”

Surprising detection of an equatorial dust lane on the AGB star IRC+10216

“…The refractive indices are taken from Preibisch et al (1993), Pitman et al (2008), and Begemann et al (1994), for the amorphous carbon, SiC, and MgS respectively. For the size distribution of the grains we use n(a)da ∝ a −3.5 , ranging from 0.005 micron up to 0.25 micron (Mathis et al 1977).…”