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ABSTRACT Polarization is a unique tool to study the dust grains of protoplanetary discs. 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 λ. Polarization at Band 7 is predominantly parallel to the disc minor axis but appears azimuthally oriented at Band 3, with the morphology at Band 6 in between the two. We present new ∼0.2 arcsec (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 disc with the most complete wavelength coverage in dust polarization. The polarization patterns at Bands 4 and 5 follow the previously identified morphological transition with wavelength. From the azimuthal variation, we decompose the polarization into contributions from scattering (s) and thermal emission (t). s decreases slowly with increasing λ, and t increases more rapidly which are expected from optical depth effects of toroidally aligned scattering prolate grains. The weak λ dependence of s is inconsistent with the simplest case of Rayleigh scattering by small grains in the optically thin limit but can be affected by factors such as optical depth, disc substructure, and dust porosity. The sparse polarization detections from the Q-band image are also consistent with toroidally aligned prolate grains. 

ABSTRACT Despite many methods developed to find young massive planets in protoplanetary discs, it is challenging to directly detect low-mass planets that are embedded in discs. On the other hand, the core-accretion theory suggests that there could be a large population of embedded low-mass young planets at the Kelvin-Helmholtz (KH) contraction phase. We adopt both 1D models and 3D simulations to calculate the envelopes around low-mass cores (several to tens of M⊕) with different luminosities, and derive their thermal fluxes at radio wavelengths. We find that, when the background disc is optically thin at radio wavelengths, radio observations can see through the disc and probe the denser envelope within the planet’s Hill sphere. When the optically thin disc is observed with the resolution reaching one disc scale height, the radio thermal flux from the planetary envelope around a 10 M⊕ core is more than 10 per cent higher than the flux from the background disc. The emitting region can be extended and elongated. Finally, our model suggests that the au-scale clump at 52 au in the TW Hydrae disc revealed by ALMA is consistent with the envelope of an embedded 10–20 M⊕ planet, which can explain the detected flux, the spectral index dip, and the tentative spirals. The observation is also consistent with the planet undergoing pebble accretion. Future ALMA and ngVLA observations may directly reveal more such low-mass planets, enabling us to study core growth and even reconstruct the planet formation history using the embedded ‘protoplanet’ population.