ALMA Polarimetry reveals the properties of protoplanetary disks with rotating jets: clues on grains

We have obtained ALMA polarimetric data at 870 microns (Band 7) and 0."2 resolution for the protoplanetary disks around DG Tau and CW Tau, two T Tauri systems with well known rotating jets. In both targets the results are consistent with dust self-scattering being at the origin of the polarization pattern (Bacciotti et al., 2018, ApJ 865, L12). The polarization maps highlight structural details not derivable from the analysis of the total emission alone. Such substructures are likely related to changes in the optical depth, and can betray a change in the physical structure of the disk. The has been confirmed by subsequent ALMA observations by our group (Podio et al., 2019, A&A 623, L6). The measured values of the polarization fraction give constraints to the maximum size of the grains giving rise to observed patterns. This size is found to be in the range 50 - 150 microns in our targets. Meanwhile, the polarization maps combined with dust opacity estimates give constrai nts on the scale height of such grains. Overall, we find that these grains are larger and more settled toward the midplane in CW Tau than in DG Tau. These results confirm that polarization can be a fundamental complement to observations in total emission, for the investigation of the structure and of the evolution of protoplanetary disks. The promising prospects of such investigations with ALMA will be discussed.