Coagulation of Ice-coated particles is limited

At a 112.4 pc distance, the PDS70 protoplanetary disk is a rare case that has been confirmed to host two accreting planets. This makes it the most important laboratory for studying dust growth in the context of planet formation. Here we present the first deep, full polarization observations at 873 micron wavelength. We detected ~1%-2.5% linear polarization over the bulk of the ~55-100 AU (sub)millimeter ring. The polarization position angles align preferentially with the projected minor axis of the disk. The standard interpretation is that the observed polarization is caused by dust self-scattering, with a maximum dust grain size of ~100 microns. On >~10 AU scales, which can be resolved by the presented 873-3075 micron observations, the ring is marginally optical thick at 873 micron wavelength. Using Monte Carlo radiative transfer simulations, we found that an azimuthally asymmetric, marginally optically thick ring with a maximum dust grain size of ~87 micron can produce the observed 873 micron polarization position angles and percentages. This study indicates that the coagulation of ice-coated dust in the protoplanetary disk is limited by fragmentation or bouncing. Sublimating ice or concentrating dust by planet-disk interaction may create environments that are prone to dust growth and planet formation.