High-resolution submillimeter and near-infrared studies of the transition disk around Sz 91

To reveal the structures of a transition disk around a young stellar object in Lupus, Sz 91 , we have performed aperture synthesis 345 GHz continuum and CO(3-2) observations with the Submillimeter Array (1″-3″ resolution) and high-resolution imaging of polarized intensity at the K_s -band using the HiCIAO instrument on the Subaru Telescope (0.″25 resolution). Our observations successfully resolved the inner and outer radii of the dust disk to be 65 and 170 AU, respectively, which indicates that Sz 91 is a transition disk source with one of the largest known inner holes. The model fitting analysis of the spectral energy distribution reveals an H₂ mass of 2.4 × 10^-3 M_⊙ in the cold (T < 30 K) outer part at 65 AU <r < 170 AU by assuming a canonical gas-to-dust mass ratio of 100, although a small amount (>3 × 10^-9 M _⊙) of hot (T180 K) dust possibly remains inside the inner hole of the disk. The structure of the hot component could be interpreted as either an unresolved self-luminous companion body (not directly detected in our observations) or a narrow ring inside the inner hole. Significant CO(3-2) emission with a velocity gradient along the major axis of the dust disk is concentrated on the Sz 91 position, suggesting a rotating gas disk with a radius of 420 AU. The Sz 91 disk is possibly a rare disk in an evolutionary stage immediately after the formation of protoplanets because of the large inner hole and the lower disk mass than other transition disks studied thus far.