Grain alignment and polarized emission from magnetized T tauri disks

The structure of magnetic fields within protostellar disks may be studied via polarimetry provided that grains are aligned with respect to the magnetic field within the disks. We explore the alignment of dust grains by radiative torque in T Tauri disks and provide predictions for polarized emission for disks viewed at different wavelengths and viewing angles. We show that the alignment is especially efficient in the outer parts of the disks. In the presence of a magnetic field, these aligned grains produce polarized emission in infrared wavelengths. We consider a simple disk model and provide predictions for polarization that are available to the present-day instruments that do not resolve the disks and will be available to future instruments that will resolve the disks. We find that the polarized emission drops for wavelengths shorter than similar to 10 mu m. Between similar to 10 and similar to 100 mu m, the polarized emission is dominated by the emission from the surface layer, and the degree of polarization can be as large as similar to 10% for unresolved disks. We find that the degree of polarization at these wavelengths is very sensitive to the size distribution of dust grains in the disk surface layer, which should allow for the testing of the predicted grain-size distributions. The degree of polarization in the far-infrared/submillimeter wavelengths is sensitive to the size distribution of dust grains in the disk interior. When we take a Mathis-Rumpl-Nordsieck-type distribution with maximum grain size of 500-1000 mu m, the degree of polarization is around the 2%-3% level at wavelengths larger than similar to 100 mu m. Our study indicates that multifrequency infrared polarimetric studies of protostellar disks can provide good insights into the details of their magnetic structure.