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 in respect to magnetic field within the disks. We explore 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 magnetic field, these aligned grains produce polarized emission in infrared wavelengths. We consider a simple model of an accretion disk 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. As the surface magnetic field and the bulk magnetic field play different roles for the disk dynamics, we consider separately the contributions that arise from the surface areas of the disk and its interior. We find that the polarized emission drops for wavelengths shorter than ∼ 10μm. Between ∼ 10μm and ∼ 100μm, the polarized emission is dominated by the emission from the surface layer of the disks and the degree of polarization can be as large as ∼ 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 testing the predicted grain size distributions. The degree of polarization in the far-infrared/sub-millimeter 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 μm, the degree of polarization is around 2-3 % level at wavelengths larger than ∼100μm. Our study indicates that multifrequency infrared polarimetric studies of protostellar disks can provide good insights into the details of their magnetic structure. Subject headings: accretion, accretion disks —circumstellar matter — polarization — stars: pre-mainsequence — dust, extinction