2020-7-15

Grain size and turbulence strength in protoplanetary disks inferred from multi-wavelength polarimetric observations

Speaker: Takahiro Ueda (NAOJ)

Abstract:

Dust grains are building blocks of rocky planets and cores of giant planets. The evolution of dust grains into larger bodies is governed by turbulent motion of disk gas because turbulence controls the kinematic motion of dust grains. The ALMA polarimetric observations have shown that many disks show the scattering-induced polarization pattern at sub-millimeter wavelength. It suggests that the dust-size evolution might be halted at ~100um. In contrast, the turbulence strength has been not well constrained directly by observations. We investigate the grain size and turbulence strength in the disk around HL Tau.
First, we give a constrain on the grain size in the disk by analyzing the spectral energy distribution (SED). The SED shows that the intensity at the ALMA Band 4 is lower than that estimated from the black body radiation even though it is optically thick. It indicates that the disk contains mm-sized grains which has high scattering opacity at the ALMA Band 4.
This result seems to contradict to the size previously predicted by the polarimetric observations which show that ~100um-sized grain is dominant. However, there is a possibility that the polarimetric observations trace only the upper layer of the disk which is dominated by ~100um-sized grains and mm-sized grains are hidden into the disk mid-plane.
By using radiative transfer simulation with the differential settling, we demonstrate that the very low turbulence (α<10-5) allows mm-sized grains to settle to the optically thick disk mid-plane and can reproduce both the SED and polarimetric observations even if mm-sized grains exist.
This approach would be a useful way to constrain grain size and turbulence strength in protoplanaetary disks.