Thermal Structure of a Protoplanetary Disk around HD163296: A Study of Vertical Temperature Distribution by CO Emission Lines

Abstract: This paper presents observations of a protoplanetary disk around Herbig Ae star HD 163296 in 12CO (J=1-0), 12CO (J=3-2), 13CO (J=1-0), and 13CO (J=3-2) emission lines. Double-peaked emission profiles originating from the rotating circumstellar disk were detected in all the lines. The disk parameters were estimated from model calculation in which the radial distribution of temperature or surface density inside the disk has a power-law form. The surface density should be sufficiently high so that the disk is optically thick for all the CO lines, as discussed in previous studies based on interferometric observations. The temperature and outer radius of the disk were also confirmed to be consistent with the previous results. Taking advantage of difference in position of the photosphere among the CO lines, we revealed temperature distribution in vertical direction. The temperature of 12CO (J=3-2) emitting region is about twice higher than that of any other CO emitting region; the former is about 59 K while the latter is about 31 K at 100 AU from the central star, suggesting that there are at least two distinct temperature regions. The best fit temperature for 13CO (J=1-0) that should trace the deepest region of the disk is even lower, implying that there is also a different temperature region in deep inside of the disk. Such vertical temperature distribution in a disk was identified both in T Tauri and Herbig Ae stars (e.g., DM Tau, AB Aur, and HD 31648), and this should be a common feature in protoplanetary disks.