Gas structure inside dust cavities of transition disks: Ophiuchus IRS 48 observed by ALMA

Context. Transition disks are recognized by the absence of emission of small dust grains inside a radius of up to several 10s of AUs. Owing to the lack of angular resolution and sensitivity, the gas content of these dust holes has not yet been determined, but is of importance for constraining the mechanism leading to the dust holes. It is thought that transition disks are currently undergoing the process of dispersal, setting an end to the giant planet formation process. Aims. We present new high-resolution observations with the Atacama Large Millimeter/submillimeter Array (ALMA) of gas lines towards the transition disk Oph IRS 48 previously shown to host a large dust trap. The ALMA telescope has detected the J = 6-5 line of (CO)-C-12 and (CO)-O-17 around 690 GHz (434 mu m) at a resolution of similar to 0.25 '' corresponding to similar to 30 AU (FWHM). The observed gas lines are used to set constraints on the gas surface density profile. Methods. New models of the physical-chemical structure of gas and dust in Oph IRS 48 are developed to reproduce the CO line emission together with the spectral energy distribution and the VLT-VISIR 18.7 mu m dust continuum images. Integrated intensity cuts and the total spectrum from models having different trial gas surface density profiles are compared to observations. The main parameters varied are the drop in gas surface density inside the dust-free cavity with a radius of 60 AU and inside the gas-depleted innermost 20 AU. Using the derived surface density profiles, predictions for other CO isotopologues are made, which can be tested by future ALMA observations of the object. Results. From the ALMA data we find a total gas mass of the disk of 1.4 x 10(-4) M-circle dot. This gas mass yields a gas-to-dust ratio of similar to 10, but with considerable uncertainty. Inside 60 AU, the gas surface density drops by a factor of similar to 12 for an assumed surface density slope of gamma = 1 (Sigma proportional to r(-r)). Inside 20 AU, the gas surface density drops by a factor of at least 110. The drops are measured relative to the extrapolation to small radii of the surface density law at radii >60 AU. The inner radius of the gas disk at 20 AU can be constrained to better than +/- 5 AU. Conclusions. The derived gas surface density profile points to the clearing of the cavity by one or more massive planets/companions rather than just photoevaporation or grain-growth.