Gas inside the 97 AU cavity around the transition disk Sz 91

We present ALMA (Cycle 0) band 6 and band 3 observations of the transition disk Sz 91. The disk inclination and position angle are determined to be i = 49.°5 ± 3.°5°and PA = 18.°2 ± 3.°5 and the dusty and gaseous disk are detected up to ∼220 and ∼400 AU from the star, respectively. Most importantly, our continuum observations indicate that the cavity size in the millimeter-sized dust distribution must be ∼97 AU in radius, the largest cavity observed around a T Tauri star. Our data clearly confirm the presence of ${}^{12}CO$ (2–1) well inside the dust cavity. Based on these observational constraints we developed a disk model that simultaneously accounts for the ${}^{12}CO$ and continuum observations (i.e., gaseous and dusty disk). According to our model, most of the millimeter emission comes from a ring located between 97 and 140 AU. We also find that the dust cavity is divided into an innermost region largely depleted of dust particles ranging from the dust sublimation radius up to 85 AU, and a second, moderately dust-depleted region, extending from 85 to 97 AU. The extremely large size of the dust cavity, the presence of gas and small dust particles within the cavity, and the accretion rate of Sz 91 are consistent with the formation of multiple (giant) planets.