An Observational Study of the Temperature and Surface Density Structures of a Typical Full Disk around MWC 480

This paper presents observations of a protoplanetary disk around a Herbig Ae star, MWC 480, in (CO)-C-12 (J = 1-0), (CO)-C-12 (J = 3-2), (CO)-C-13 (J = 1-0), and (CO)-O-18 (J = 1-0) emission lines. Double-peaked emission profiles originating from the rotating circumstellar disk were detected in all of the lines. The vertical temperature and radial surface density structures of the outer region of the disk were derived by applying the similarity solution in the standard accretion disk model. Taking advantage of differences in the height of the photosphere among the CO lines, the temperature in the uppermost (CO)-C-12 (J = 3-2) emitting layer was shown to be about 3-times higher than that of any other CO emitting region, suggesting that there are at least two distinct temperature regions. Our modeling succeeds in describing all of the observational results obtained in the four CO lines, particularly different emission extents at different frequencies, by a single set of the parameters for a disk model. Since the similarity solution model could be the most suitable for the radial surface density structure, it is likely that the disk around MWC 480 evolves by transferring angular momentum outward via viscous diffusion. Although further quantitative studies are required for identifying what disk model is the best for describing physical disk structures, our results suggest the potential advantage of the similarity solution model, indicating that disks around Herbig Ae/Be stars likely have diffused gas in the outer regions, and that the disk surface density exponentially decreases with increasing radial distance.