2019ApJ...871....5T

We report 0.''14 resolution observations of the dust continuum at band 7, and the CO(3-2) and HCO⁺(4-3) line emissions toward the transitional disk around Sz 91 with the Atacama Large Millimeter/submillimeter Array. The dust disk appears to be an axisymmetric ring, peaking at a radius of ∼95 au from a Gaussian fit. The Gaussian fit widths of the dust ring are 24.6 and 23.7 au for the major and the minor axes, respectively, indicating that the dust ring is not geometrically thin. The gas disk extends out to ∼320 au and is also detected in the inner hole of the dust ring. A twin-line pattern is found in the channel maps of CO, which can be interpreted as the emission from the front and rear of the flared gas disk. We perform radiative transfer calculations using RADMC-3D to check whether the twin-line pattern can be reproduced assuming that the flared gas disk has a power-law form for the column density and T_gas = T_dust. The thermal Monte Carlo calculation in RADMC-3D shows that the disk temperature has a gradient along the vertical direction beyond the dust ring, as it blocks the stellar radiation, and thus the twin-line pattern can be naturally explained by the flared gas disk in combination with the dust ring. In addition, no significant depletion of the CO molecules in the cold midplane achieves a reasonable agreement with the observed twin-line pattern. This result indicates that the CO emission from the rear surface must be heavily absorbed in the cold midplane.