Evidence of Grain Alignment by Magnetically Enhanced Radiative Torques from Multiwavelength Dust Polarization Modeling of HL Tau

TL;DR

This study models dust polarization in HL Tau using magnetically enhanced radiative torques and scattering, revealing the importance of iron inclusions and grain size constraints across multiple wavelengths.

Contribution

It provides the first detailed modeling of dust polarization based on grain alignment physics, incorporating magnetic effects and iron inclusions to match observations.

Findings

Iron inclusions in grains are at least 16% by volume.

Maximum grain sizes are approximately 60, 80, and 90 micrometers at 0.87, 1.3, and 3.1 mm wavelengths.

Grain alignment and composition are crucial for reproducing observed polarization patterns.

Abstract

Atacama Large Millimeter/Submillimeter Array (ALMA) has revolutionized the field of dust polarization in protoplanetary disks across multiple wavelengths. Previous observations and empirical modeling suggested multiple mechanisms of dust polarization toward HL Tau, including grain alignment and dust scattering. However, a detailed modeling of dust polarization based on grain alignment physics is not yet available. Here, using our updated POLARIS code, we perform numerical modeling of dust polarization arising from both grain alignment by Magnetically Enhanced Radiative Torque (MRAT) mechanism and self-scattering to reproduce the HL Tau polarization observed at three wavelengths 0.87, 1.3, and 3.1$\,$mm. Our modeling results show that the observed multi-wavelength polarization could be reproduced only when large grains contain embedded iron inclusions and those with slow internal relaxation must have wrong internal alignment (i.e., the grain's major axis parallel to its angular momentum). The abundance of iron embedded inside grains in the form of clusters is constrained to be $\gtrsim 16$%, and the number of iron atoms per cluster is $N_{\rm cl} \sim 9\times10^2$. Maximum grain sizes probed at wavelengths $\lambda$ = 0.87, 1.3, and 3.1$\,$mm are constrained at $\sim$ 60, 80, and 90$\,\mu$m, respectively.