Polarization of dust emission in clumpy molecular clouds and cores

Grain alignment theory has reached the stage where quantitative predictions of the degree of alignment and its variations with optical depth are possible. With the goal of studying the effect of clumpiness on submillimeter and far-infrared polarization, we have computed the polarization due to alignment via radiative torques within clumpy models of cores and molecular clouds. Our models were based on a highly inhomogeneous simulation of compressible MHD turbulence. A reverse Monte Carlo radiative transfer method was used to calculate the intensity and anisotropy of the internal radiation field, and the subsequent grain alignment was computed for a power-law size distribution of grains using the DDSCAT package for radiative torques. The intensity and anisotropy of the intra-cloud radiation field show large variations throughout the models but are generally sufficient to drive widespread grain alignment. The P-I relations for our models reproduce those seen in observations. We show that the degree of polarization observed is extremely sensitive to the upper grain size cutoff and is less sensitive to changes in the radiative anisotropy. Furthermore, despite a variety of dust temperatures along a single line of sight through our models and among dust grains of different sizes, the assumption of isothermality among the aligned grains does not introduce a significant error. Our calculations indicate that submillimeter polarization vectors can be reasonably good tracers for the underlying magnetic field structure, even for relatively dense clouds ( A(V) similar to 10 to the cloud center). The current predictive power of the grain alignment theory should motivate future polarization observations using the next generation of multiwavelength submillimeter polarimeters such as those proposed for SOFIA.