[P-PS06] Planetary Sciences

[PPS06-07] ALMA high resolution observations of the transitional disk around SY Cha.

*Ryuta Orihara¹, Munetake Momose¹, Takayuki Muto², Jun Hashimoto³, Hauyu Baobab Liu⁴, Takashi Tsukagoshi⁵, Tomoyuki Kudo⁵, Sanemichi Takahashi⁵, Yi Yang^5,6, Yasuhiro Hasegawa⁷, Ruobing Dong⁸, Mihoko Konisi⁹, Eiji Akiyama¹⁰, Masato Ishiduka⁶ (1.Ibaraki University, 2.Kogakuin University, 3.Astrobiology Center, 4.Academia Sinica Institute of Astronomy and Astrophysics, 5.National Astronomical Observatory of Japan, 6.University of Tokyo, 7.NASA Jet Propulsion Laboratory, 8.University of Victoria, 9.Oita University, 10.Niigata institute of technology)

Keywords:ALMA, protoplanetary disk, transitional disk, warp

Transitional disks have a cavity with weak dust emission in the central region, and they are thought to be highly evolved protoplanetary disks. The formation mechanism of such disks is still not known. Transitional disks are identified as objects with low infrared excess in the spectral energy distribution (SED). However, it is difficult to classify transitional disks only with SED. There is a class of disks so-called "pre-transitional disks", which has a small inner disk in the cavity of the outer transitional disk. These are considered to be precursors to the transitional disks and therefore are important in understanding disk evolution processes. Since the SED of a pre-transitional disk is indistinguishable from that of a transitional disk, high-resolution observations that reveal the actual structure of gas and dust at a small scale are necessary to study disk evolution and planet formation.
In this presentation, we report the results of high-resolution (0.14 arcsec) observations of the 225 GHz dust continuum and CO molecular emission lines from SY Cha (distance: 180.7 pc, mass: 0.5-0.7 Msun, age: 3 million years), which is selected from a sample of objects showing transitional disk features in their SEDs. In the dust continuum emission, a ring structure with the radius of about 100 au is detected, as well as a central point source that has not been identified before from SED analyses. Our high resolution observations clearly reveal the inner cavity and the central point source for the first time. We fit the radial profiles of dust continuum emission and find that there is a Gaussian ring of 35 au in radial width on top of another flattened ring with the radial width of 135 au. The reduced chi-square value for the best-fit model is 1.38, which is calculated for actual visibility data in the processes of MCMC fitting. We also find that there is a weak asymmetry in dust continuum emission.
In our observations, 12CO(2-1) and 13CO(2-1) emission are also detected. We estimate the surface density of gas and find that the total gas mass is comparable with that of dust, indicating that the gas is highly depleted. We also find that the gas is present inside the dust cavity. The analysis of the velocity structure of the 12CO(2-1) emission line suggests that the velocity is distorted at the location of the dust inner disk. This may be due to warp of the disk or radial gas flow within the cavity of the dust disk. In our presentation, we report the basic properties of the transitional disk system associated with SY Cha derived from the observations.