Panchromatic (Sub)millimeter Polarization Observations of HL Tau Unveil Aligned Scattering Grains

TL;DR

This study provides comprehensive multi-wavelength polarization observations of HL Tau, revealing the transition from scattering-dominated to thermal emission-dominated polarization, indicating large, porous, aligned dust grains in the disk.

Contribution

It presents the most complete wavelength coverage of polarization in HL Tau, combining VLA and ALMA data to analyze dust grain alignment and scattering effects.

Findings

Polarization morphology varies with wavelength, transitioning from azimuthal to minor axis alignment.

Scattering contribution decreases slowly with wavelength, consistent with large, porous grains.

Thermal emission increases rapidly with wavelength, indicating optical depth effects.

Abstract

Polarization is a unique tool to study the properties of dust grains of protoplanetary disks and detail the initial conditions of planet formation. Polarization around HL Tau was previously imaged using the Atacama Large Millimeter/submillimeter Array (ALMA) at Bands 3 (3.1 mm), 6 (1.3 mm), and 7 (0.87 mm), showing that the polarization orientation changes across wavelength $\lambda$. The polarization morphology at Band 7 is predominantly parallel to the disk minor axis but appears azimuthally oriented at Band 3, with the morphology at Band 6 in between the two. We present new ~0.2" (29 au) polarization observations at Q-Band (7.0 mm) using the Karl G. Jansky Very Large Array (VLA) and at Bands 4 (2.1 mm), 5 (1.5 mm), and 7 using ALMA, consolidating HL Tau's position as the protoplanetary disk with the most complete wavelength coverage in dust polarization. The polarization patterns at Bands 4 and 5 continue to follow the morphological transition with wavelength previously identified in Bands 3, 6, and 7. Based on the azimuthal variation, we decompose the polarization into contributions from scattering ($s$) and thermal emission ($t$). We find that $s$ decreases slowly with increasing $\lambda$, and $t$ increases more rapidly with $\lambda$ which are expected from optical depth effects of toroidally aligned, scattering prolate grains. The relatively weak $\lambda$ dependence of $s$ is consistent with large, porous grains. The sparse polarization detections from the Q-band image are also consistent with toroidally aligned prolate grains.