Infrared extinction and polarization due to partially aligned spheroidal grains - Models for the dust toward the BN object

Abstract

Methods for computing the extinction and polarization due to partially aligned, precessing grains in the dipole aproximation regime are described. The methods are employed to develop theoretical models of the dust grains on the line of sight to the Becklin-Neugebauer (BN) object in the dense molecular cloud OMC-1. In the models, it is assumed that the dust grains consist of a mixture of graphite and silicate particles, and that some of the particles are coated with ice mantles. It is shown that the models are capable of reproducing all the spectroscopic and polarimetric features of the dust grains in the spectral range from about 2 to about 13 microns. The implications of the grain models for the sizes, shapes and compositions of grains on the line-of-sight to BN are discussed, and some possible mechanisms for grain alignment are identified.