title: Radial Density Structure of Protoplanetary Disk: Power-Law Disk Model vs. Similarity Solution Disk Model

authors: Eiji Akiyama, Munetake Momose, Takashi Tsukagoshi, and Yoshimi Kitamura

abstract: With the recent advent of improved observational equipment and facilities, the observation of diffused gas near outer edge of the disk that used to be very difficult to observe has recently become possible. We made multi-CO line observations toward a well-known object, HD 31648 (MWC 480), in millimeter and sub-millimeter wavelengths by NRO 45 m and ASTE 10 m telescopes, and investigate the radial density structure near outer edge of the disk by comparing two disk models: the power-law disk model with truncation in the radial surface density distribution at a particular outer radius and the similarity solution disk model with a tapered outer edge. The emission from the tenuous gas in the outer disk that makes a peak-to-peak velocity width narrower was detected in the optically thick line, whereas the emission from the dense inner disk that makes the width wider was detected in the optically thin line. As a result of comparison between both disk models, the observed profiles show a good agreement with the ones derived by the similarity solution disk model. It indicates that the disk gas extends and gradually tapers off. It is consistent with the model fitting results improved by the similarity solution disk model toward other star+disk systems, such as HD 163296 and GM Aurigae in the continuum and molecular gas observations. Although it requires further validation, it seems that the tapered outer edge structure is ubiquitous in protoplanetary disks.