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Recent (sub)millimeter polarimetric observations toward the young star HL Tauhave successfully detected polarization emission from its circumstellar disk.The polarization pattern observed at 0.87 mm is uniform and parallel to thedisk's minor axis, consistent with the self-scattering of thermal emission bydust particles whose maximum radius is $\approx 100~\mu m$. However, thismaximum size is considerably smaller than anticipated from dust evolutionmodels that assume a high sticking efficiency for icy particles. Here, wepropose that the unexpectedly small particle size can be explained if CO$_2$ice covers the particles in the outer region of the HL Tau disk. CO$_2$ ice isone of the most major interstellar ices, and laboratory experiments show thatit is poorly sticky. Based on dust evolution models accounting for CO$_2$ icemantles as well as aggregate sintering and post-processing radiative transfer,we simulate the polarimetric observation of HL Tau at 0.87 mm. We find that themodels with CO$_2$ ice mantles better match the observation. These models alsopredict that only particles lying between the H$_2$O and CO$_2$ snow lines cangrow to millimeter to centimeter sizes, and that their rapid inward driftresults in a local dust gap similar to the 10 au gap of the HL Tau disk. Wealso suggest that the millimeter spectral index for the outer part of the HLTau disk is largely controlled by the optical thickness of this region and doesnot necessarily indicate dust growth to millimeter sizes.

Nonsticky Ice at the Origin of the Uniformly Polarized Submillimeter Emission from the HL Tau Disk