Millimeter-wave polarization due to grain alignment by the gas flow in protoplanetary disks

Abstract

Dust grains emit intrinsic polarized emission if they are elongated and aligned in the same direction. The direction of the grain alignment is determined by external forces, such as magnetic fields, radiation, and gas flow against the dust grains. In this letter, we apply the concept of the grain alignment by gas flow, which is called mechanical alignment, to the situation of a protoplanetary disk. We assume that grains have a certain helicity, which results in the alignment with the minor axis parallel to the grain velocity against the ambient disk gas and discuss the morphology of polarization vectors in a protoplanetary disk. We find that the direction of the polarization vectors depends on the Stokes number, which denotes how well grains are coupled to the gas. If the Stokes number is less than unity, orientation of polarization is in the azimuthal direction since the dust velocity against the gas is in the radial direction. If the Stokes number is as large as unity, the polarization vectors show a leading spiral pattern since the radial and azimuthal components of the gas velocity against the dust grains are comparable. This suggests that if the observed polarization vectors show a leading spiral pattern, it would indicate that Stokes number of dust grains is around unity, which is presumably radially drifting.

7 pages, 5 figures, accepted for publication in The Astrophysical Journal Letters

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Cited by in corpus (12)

• Nonsticky Ice at the Origin of the Uniformly Polarized Submillimeter Emission from the HL Tau Disk
• Radial variations in grain sizes and dust scale heights in the protoplanetary disk around HD 163296 revealed by ALMA polarization observation
• Polarized emission by aligned grains in the Mie regime : application to protoplanetary disks observed by ALMA
• Modeling of the ALMA HL Tau Polarization by Mixture of Grain Alignment and Self-scattering
• Observationally derived magnetic field strength and 3D components in the HD 142527 disk
• Compaction during fragmentation and bouncing produces realistic dust grain porosities in protoplanetary discs
• Thermal Emission and Scattering by Aligned Grains: Plane-Parallel Model and Application to Multiwavelength Polarization of the HL Tau Disk
• Probing the Temperature Structure of Optically Thick Disks Using Polarized Emission of Aligned Grains
• Wavelength-dependent far-infrared polarization of HL Tau observed with SOFIA/HAWC+
• Silicate Sundogs: Probing the Effects of Grain Directionality in Exoplanet Observations
• Exploring Polarized Millimeter Emission from Protoplanetary Disks with Irregular Dust Grains
• History of the Solar Nebula from Meteorite Paleomagnetism