Investigation of the polarization observed in infrared absorption bands in the spectra of protostars

We investigate the linear polarization in the two deepest infrared absorption bands observed in the spectra of protostars, the water-ice band with the center near 3.1 mu m and the silicate band with the center near 9.7 mu m, using a core-mantle confocal spheroid model with various axial ratios a/b and relative volumes of the core material. We consider the effect of the grain shape, structure, and type (oblate, prolate) as well as the type of grain orientation and its location relative to the incident ray of light and the magnetic field direction on the central wavelengths of the two bands and the polarizability in the bands. We have found that the observed relationships between the polarizability in the bands and the ratio of their optical depths at the band centers can be explained if we choose slightly oblate or prolate particles (a/b less than or similar to 2 for the silicate band and 1.3 less than or similar to a/b less than or similar to 2 for the ice band). For any type of orientation, the core-mantle confocal spheroid model requires different axial ratios for the ice and silicate bands to account for the observed polarization. We show that picket-fence-oriented particles can explain the observed polarization in the ice band at angles alpha between the particle rotation axis and the incident ray greater than or similar to 30 degrees and in the silicate band at any alpha. Perfectly Davis-Greenstein-oriented particles can explain the observed polarization in the ice band at angles Omega between the line of sight and the magnetic field direction greater than or similar to 60 degrees and in the silicate band at any Omega. The orientation parameter xi (imperfect Davis-Greenstein orientation) must be no more than 0.5 (oblate particles) and 0.1 (prolate particles) for the ice band and can be arbitrary for the silicate band.