Grain Alignment and Polarized Emission from Magnetized T Tauri Disks

Abstract

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 $10 m. Between $10 and $100 m, the polarized emission is dominated by the emission from the surface layer, 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 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 MathisRumpl-Nordsieck-type distribution with maximum grain size of 500-1000 m, the degree of polarization is around the 2%-3% level at wavelengths larger than $100 m. Our study indicates that multifrequency infrared polarimetric studies of protostellar disks can provide good insights into the details of their magnetic structure.

Detection of a Magnetized Disk Around a Very Young Protostar

“…These upper limits are well below the ≃ 2-3% polarization fractions expected from simulations of a magnetized circumstellar disk (i.e., having a predominantly toroidal magnetic field) with standard dust grain alignments (Cho & Lazarian 2007). These results have been interpreted either due to inefficient alignment of large grains, or insufficient magnetic field strength for alignment, or as a consequence of magnetic field tangling by turbulent motions (Hughes et al 2013).…”

First Results from BISTRO: A SCUBA-2 Polarimeter Survey of the Gould Belt

“…B-field geometries are generally inferred by preferential emission or absorption by dust or molecules, creating polarized light (e.g., Houde et al 2004Houde et al , 2013Cho & Lazarian 2007). Polarization measurements with molecules require bright lines and are generally restricted to very densesmall-scale structures.…”

Submillimeter Array Observations of Magnetic Fields in Star Forming Regions

“…SMA observations of the disk around HD 163296 (Figure 3; Hughes et al 2009), do not detect any polarized emission from either disk. The observations set the most stringent limits to date on the millimeter wavelength polarization from protoplanetary disks, and rule out the fiducial Cho & Lazarian (2007) model at the 10-sigma level. By comparing the SMA observations to the model predictions of Cho & Lazarian (2007), it is determined that the factors most likely contributing to the suppression of polarized emission relative the fiducial model are the roundness of large grains, inefficient alignment of grains with the magnetic field, and a random "tangled" component to the magnetic field lines.…”

“…Previous observations (Tamura et al, 1999) and theoretical predictions (Cho & Lazarian 2007) have suggested that a polarization fraction of 2-3% should be commonly observed for protoplanetary disks around young stars. SMA observations of the disk around HD 163296 (Figure 3; Hughes et al 2009), do not detect any polarized emission from either disk.…”