The Radial Distribution of the Surface Density of a Protoplanetary Disk Around HD 163296: Evidence for the Viscous Evolution of the Disk

We present observations of a protoplanetary disk around a Herbig Ae star HD 163296 obtained from the ALMA Science Verification data, as well as the Nobeyama 45 m and the ASTE 10 m single dish telescope observations. Double-peaked emission profiles from the rotating circumstellar disk were detected in a number of CO isotopologue lines. The radial surface density structures of the outer region of the disk were examined by comparing the similarity solution model with an exponential tapered edge with the power-law disk model having a truncated outer edge. As a result, the exponential taper description effectively reproduces the observational data, indicating that the diffuse gas in the outer region of the disk tapers off gradually. Since the similarity solution model is based on the dynamical evolution of a viscous disk, the disk around HD 163296 likely evolves by transferring angular momentum outwards via viscous diffusion.