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The mechanism for producing polarized emission from protostellar disks at(sub)millimeter wavelengths is currently uncertain. Classically, polarizationis expected from non-spherical grains aligned with the magnetic field.Recently, two alternatives have been suggested. One polarization mechanism iscaused by self-scattering from dust grains of sizes comparable to thewavelength while the other mechanism is due to grains aligned with their shortaxes along the direction of radiation anisotropy. The latter has recently beenshown as a likely mechanism for causing the dust polarization detected in HLTau at 3.1 mm. In this paper, we present ALMA polarization observations of HLTau for two more wavelengths: 870 $\mu$m and 1.3 mm. The morphology at 870$\mu$m matches the expectation for self-scattering, while that at 1.3 mm showsa mix between self-scattering and grains aligned with the radiation anisotropy.The observations cast doubt on the ability of (sub)millimeter continuumpolarization to probe disk magnetic fields for at least HL Tau. By showing twodistinct polarization morphologies at 870 $\mu$m and 3.1 mm and a transitionbetween the two at 1.3 mm, this paper provides definitive evidence that thedominant (sub)millimeter polarization mechanism transitions with wavelength. Inaddition, if the polarization at 870 $\mu$m is due to scattering, the lack ofpolarization asymmetry along the minor axis of the inclined disk implies thatthe large grains responsible for the scattering have already settled into ageometrically thin layer, and the presence of asymmetry along the major axisindicates that the HL Tau disk is not completely axisymmetric.

ALMA Reveals Transition of Polarization Pattern with Wavelength in HL Tau's Disk