Diffuse band excitation and polarization in core-mantle grains

Diffuse band shapes in both extinction and polarization are calculated for interstellar core-mantle particles for varying size distributions of mantle thickness. It is shown that no matter whether the source of the bands is in the silicate cores or the accreted icy mantles the polarization shapes are highly asymmetric for all mantle thickness. The extinction band shapes are significantly less asymmetric although the effect is clearly present. The only apparent possibility for producing symmetric band shapes in the dust grains is in the very small bare particles in interstellar space which, if they are aligned and produce the lambda 2200 band, must exhibit a strong polarization effect in this region. (Auth.)