Circular polarization by scattering from spheroidal dust grains

Large degrees of circular polarization at near-infrared wavelengths have be en reported in the OMC1 star-forming region. This discovery, in combination with compelling evidence for the existence of non-spherical aligned grains in star formation regions, has prompted us to investigate scattering from spheroidal particles as a possible mechanism for the production of large ci rcular polarization in reflection nebulae. We use a dipole calculation to m odel the small particle limit and a T-matrix code to treat arbitrarily size d particles. We find that size distributions of perfectly aligned spheroids , with only modest 2:1 axis ratios, are capable of producing circular polar ization of up to 50 per cent when scattering unpolarized incident light. Th is is the case even for dielectric materials, such as 'astronomical silicat e', as long as sufficient large particles are included in the size distribu tion. We consider the effects of particle alignment and find that spinning oblate spheroids should be much more efficient circular polarizers than equ ivalent prolate spheroids.