Abstract
The thesis to be proved is that the mechanism of optical interstellar polarization, preferential extinction by magnetically aligned
aspherical grains, can be extrapolated successfully to explain the
large infrared polarizations observed in molecular cloud protostellar
sources. Observations of the linear polarization of 13 sources between 1.65 μm and 4.8 μm and of the circular polarization of 9 sources
at 2.2 μm have been obtained. Analogously to the optical interstellar
case, the data have been modelled using the Rayleigh approximation to
calculate extinction and phase -lag efficiencies for a number of grain
models. The models successfully account for the high infrared ellipticities (ratio of linear to circular polarization) observed in the
molecular clouds, with twists in the grain alignment of about 40°;
smaller than those required to explain optical interstellar circular
polarization. A shortcoming in the model linear polarization at
X < 3 μm is attributable to a failure in the Rayleigh approximation.
A dissimilarity in the polarization through the ice band between two
of the protostellar sources can be understood by differing grain
compositions. A correlation between the position angles of polarization of the protostars and the nearby interstellar field stars can
be interpreted by saying the Galactic magnetic field permeates the dense
molecular clouds. There are indications that the polarization
mechanism even operates in the enhanced density regions of the clouds;
then a more efficient alignment mechanism than paramagnetic relaxation is required. It is suggested that this may be "pinwheeling,"
possibly accompanied by super-paramagnetism of the grains. The
twists in the magnetic field lines implied by the model may arise in
the collapse process of the rotating, magnetized clouds.
