The interpretation of observations of ice and silicate ir features and polarization across it in the spectra of protostellar objects BN and AFGL 2591 |
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| Authors: | Alex E Il'In |
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| Institution: | (1) Pulkovo Observatory, 196140 St. Petersburg, Russia |
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| Abstract: | The absorption features of ice at 3µm and of silicate at 10 and 20µm as well as the linear polarization across it have been calculated. The interpretations of data for protostellar objects BN and AFGL 2591 are made.The model of partially aligned spinning spheroidal grains with the Purcell's suprathermal alignment mechanism and the power law size distribution are considered. Core-mantle, porous and composite particles are investigated in the Rayleigh approximation. In the case of composite and porous grains the effective refractive indices are computed with the approximate Bruggeman rule.The influence of grain chemical composition, elongation and structure has been investigated. The distinctions in polarization between composite and core-mantle models are found. The mixing of grain materials smooths the individual spectral features of chemical components. The adding of graphite causes the shift of the 10µm peak position to shorter wavelengths. When the fraction of graphite increases, the peak at 20µm decreases and shifts to shorter wavelengths. The increase of elongation reduces the negative polarization at the 3µm feature, shifts the 10µm and 20µm peak positions to longer wavelengths, and raises the strength of the 20µm band. The porosity leads to similar effects.It was found that the studied grain models are in good agreement with observational data for BN and AFGL 2591 objects. The absence of polarization excess near 3µm for AFGL 2591 is attributable to a decrease in elongation of large grains as a result of coagulation. The attention is attracted to the problem of real distribution of the interstellar dust grains over the elongation parameter. The determination this distribution function and the study of its evolution in the processes of accretion and coagulation are necessary. |
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