Size limit of superparamagnetic inclusions in dust grains and difficulty of magnetic grain alignment in protoplanetary disks

Abstract

Alignment of non-spherical grains with magnetic fields is an important problem as it lays the foundation of probing magnetic fields with polarized dust thermal emissions. In this paper, we investigate the feasibility of magnetic alignment in protoplanetary disks (PPDs). We use an alignment condition that Larmor precession should be fast compared with the damping timescale. We first show that the Larmor precession timescale is some three orders of magnitude longer than the damping time for millimeter-sized grains under conditions typical of PPDs, making the magnetic alignment unlikely. The precession time can be shortened by superparamagnetic inclusions (SPIs), but the reduction factor strongly depends on the size of the SPI clusters, which we find is limited by the so-called "Néel's relaxation process." In particular, the size limit of SPIs is set by the so-called "anisotropic energy constant" of the SPI material, which describes the energy barrier needed to change the direction of the magnetic moment of an SPI. For the most common iron-bearing materials, we find maximum SPI sizes corresponding to a reduction factor of the Larmor precession timescale of order $10^3$. We also find that reaching this maximum reduction factor requires fine-tuning on the SPI sizes. Lastly, we illustrate the effects of the SPI size limits on magnetic alignment of dust grains with a simple disk model, and we conclude that it is unlikely for relatively large grains of order 100 $\mu$m or more to be aligned with magnetic fields even with SPIs.

13 pages, 4 figures, accepted for publication on AAS journals

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• On Internal and External Alignment of Dust Grains in Protostellar Environments
• Physical Modeling of Dust Polarization from Magnetically Enhanced Radiative Torque (MRAT) Alignment in Protostellar Cores with POLARIS
• Polarization from Aligned Dust Grains in the $\beta$ Pic Debris Disk
• On Far-Infrared and Submm Circular Polarization
• Physical conditions for dust grain alignment in Class 0 protostellar cores II. The role of the radiation field in models aligning/disrupting dust grains
• The Transition of Polarized Dust Thermal Emission from the Protostellar Envelope to the Disk Scale
• Synthetic Modelling of Polarized Dust Emission in Intermediate-Mass YSOs: I: Constraining the Role of Iron Inclusions and Inelastic Relaxation on Grain Alignment with ALMA Polarization
• Probing Magnetic Fields in Protoplanetary Disk Atmospheres through Polarized Optical/IR Light Scattered by Aligned Grains
• Modeling the Far-Infrared Polarization Spectrum of a High-Mass Star Forming Cloud
• Revealing magnetic field structure at the surfaces of protoplanetary disks via near-infrared circular polarization