Dust Growth and Settling in Protoplanetary Disks and Evolution of Disk Spectral Energy Distributions
          Hidekazu Tanaka, Tokyo Institute of Technology
          Shigeru Ida, Tokyo Institute of Technology
          Dust growth and settling in protoplanetary disks considerably affect their spectral energy distributions (SEDs). We investigated dust growth and settling through numerical simulations to examine time evolution of the disk optical thickness and SEDs. Evolution of grains is divided into a growth stage and a subsequent settling stage. At the end of the growth stage, most of large grains settle to the mid-plane of gaseous disks to form a dust layer, while small grains remain floating above the layer. The floating small grains settle to the dust layer slowly in the settling stage. It takes typically 10^6yr for micron-sized grains. The optical thickness is governed by the floating small grains rather than large grains in the dust layer. Rapid grain growth in the inner part of disks makes the radial distribution of the disk optical thickness is less steep than that of the disk surface density. We found that the radial distribution of the optical thickness is almost flat for all wavelengths at t < 10^6yr. At t > 10^6yr, the inner disk (< a few AU) becomes optically thin for the central star's light, which will be observed as an inner hole. We further examined time-evolution of disk SEDs, using our numerical results and the two-layer model. The grain growth and settling decrease the magnitude of the SEDs especially at wavelengths longer than 100 micron. Our results indicate that the decrease in the observed energy fluxes at millimeter/sub-millimeter wavelengths with the time scale of 106-10^7yr can be explained by grain growth and settling without depletion of the disks.