CAOSP abstracts, Volume: 34, No.: 3, year: 2004

Abstract: Interstellar extinction curves have a typical so-called bump at a constant wavelength of about 220 nm. This indicates that cosmic dust particles distributed in space must be quite small in comparison with the wavelengths of visible radiation. The well-known Mie theory, or its approximations, are usually employed to simulate an interaction of electromagnetic radiation with such particles. However, the conventional Mie theory is applicable only for spherical and homogeneous particles, and, as known, the spherical geometry is very rare in space. Utilization of any approximation in solving the inverse problem for interstellar extinction may therefore lead to questionable results. To evaluate possible differences between retrieved size distributions, we performed a benchmark of three various techniques. The first one is based on the anomalous diffraction approximation and offers a semi-analytical solution. The profile of an extinction curve is scalable: a simple parametrization uses the modified gamma function as a substitute for the real distribution. The second approach extends the first one, but the distribution function is not expressed in an analytical form. The final profile of size distribution is computed using Mellin's transform of kernel of the integral equation. The third solution follows the modified Tikhonov's regularization and can be applied to both spherical and non-spherical particles. There is no requirement placed on a distribution function. It is shown that direct consequences of the above discussed approximations are: i) underestimation of the amount of large particles, ii) a reduced value of the modal radius of the retrieved size distribution, and iii) quite narrow distrubution functions.

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Last update: November 26, 2004