Laboratory Infrared Optical Constants and Reflectance Spectra of Silicon Carbide

Details Laboratory Infrared Optical Constants and Reflectance Spectra of Silicon Carbide Laboratory Infrared Optical Constants and Reflectance Spectra of Silicon Carbide

Dec 2006

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006aas...209.0602p&link_type=abstract

2007 AAS/AAPT Joint Meeting, American Astronomical Society Meeting 209, #06.02; Bulletin of the American Astronomical Society, V

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The observed SiC features in astronomical IR spectra of carbon stars (C-stars) correlate with thin-film IR absorption spectra of β-SiC, the polytype most commonly found as presolar grains in meteorites. Comparison between spectra of astronomical sources and laboratory compounds alone is also not sufficient to assess the relative contributions of different minerals to a given observed spectrum: radiative transfer modeling must be performed (cf. Thompson et al. 2006, ApJ, 652, in press). For C-star spectra, radiative transfer modeling is impeded by the lack of trustworthy SiC optical constants for both β and α polytypes. To address the need for improved dust composition parameters, we measured midand far-infrared room temperature reflectance spectra for several polytypes and orientations (E perpendicular to c, E parallel to c) of commercially manufactured SiC: semiconductor grade purity 3C (β-)SiC, several colors of 6H (α-SiC), and synthetic moissanite (α-SiC). The extremely high reflectivity was connected with discrepancies existing among previous absorption laboratory spectra from thin films, crystallites, and powders. We extracted the real and imaginary parts of the complex refractive index (m(λ) = n(λ) + ik(λ)) from these data using classical dispersion analyses [Spitzer et al. 1962] and supplied these results to 1-D radiative transfer models (DUSTY; Ivezic & Elitzur 1995; Nenkova et al. 2000) to determine how the emerging spectrum should change in response to our n(λ) and k(λ) and other dust shell parameters (effective stellar temperature, inner dust shell temperature, optical depth). The results of this work have direct application to carbon-rich AGB stellar outflows, novae, supernovae, and potentially proto-planetary nebulae and may further our understanding of the contribution of SiC to carbon star spectra and the lack of SiC features in the ISM.
Work supported by NASA APRA04-000-0041, NSF-AST 0607418, and performed under contract to NASA.

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