"ALMA polarization of HL Tau - investigating the planet formation"
Abstract: Constraining the grain size in protoplanetary disks is a key to understanding the first stage of planet formation. The grain size has been estimated by measuring the spectral index at millimeter wavelengths, while it has huge uncertainties. We have proposed that millimeter-wave polarization is another method to constrain the grain size. We show that thermal dust emission is scattered off of other dust grains and the residual polarization is up to 2.5 %, which is detectable with ALMA. This self-scattering polarization is efficient only if the maximum grain size is comparable to the wavelengths. Therefore, we can constrain the grain size from millimeter-wave polarization of protoplanetary disks. Then, we have observed the protoplanetary disk around HL Tau with ALMA polarization mode at 3.1 mm, and found that the disk shows the azimuthal polarization vectors. The vectors are completely different from the polarization vectors at 1.3 mm previously obtained with CARMA. We interpret that the 3.1 mm polarization is dominated by the grain alignment with radiation fields, and 1.3 mm by the self-scattering. By modeling the wavelength dependence, the grain size is constrained to be 70 micron.