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

Abstract

Recent (sub)millimeter polarimetric observations toward the young star HL Tau have successfully detected polarization emission from its circumstellar disk. The polarization pattern observed at 0.87 mm is uniform and parallel to the disk's minor axis, consistent with the self-scattering of thermal emission by dust particles whose maximum radius is $\approx 100~\mu m$. However, this maximum size is considerably smaller than anticipated from dust evolution models that assume a high sticking efficiency for icy particles. Here, we propose 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 is one of the most major interstellar ices, and laboratory experiments show that it is poorly sticky. Based on dust evolution models accounting for CO$_2$ ice mantles 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 the models with CO$_2$ ice mantles better match the observation. These models also predict that only particles lying between the H$_2$O and CO$_2$ snow lines can grow to millimeter to centimeter sizes, and that their rapid inward drift results in a local dust gap similar to the 10 au gap of the HL Tau disk. We also suggest that the millimeter spectral index for the outer part of the HL Tau disk is largely controlled by the optical thickness of this region and does not necessarily indicate dust growth to millimeter sizes.