2009A&A...501..269P

We study the distribution and physical properties of molecular gas in the disc around the T Tauri star IM Lup on scales close to 200AU. We investigate how well the gas and dust distributions compare and work towards a unified disc model that can explain both gas and dust emission. ¹²CO, ¹³CO, and C¹⁸O J=2-1 line emission, as well as the dust continuum at 1.3mm, is observed at 1.8" resolution towards IM Lup using the Submillimeter Array. A detailed disc model based on the dust emission is tested against these observations with the aid of a molecular excitation and radiative transfer code. Apparent discrepancies between the gas and dust distribution are investigated by adopting simple modifications to the existing model. The disc is seen at an inclination of 54°±3° and is in Keplerian rotation around a 0.8-1.6M_☉ star. The outer disc radius traced by molecular gas emission is 900AU, while the dust continuum emission and scattered light images limit the amount of dust present beyond 400AU and are consistent with the existing model that assumes a 400AU radius. Our observations require a drastic density decrease close to 400AU with the vertical gas column density at 900AU in the range of 5x10²⁰-10²²cm^–2. We derive a gas-to-dust mass ratio of 100 or higher in disc regions beyond 400AU. Within 400AU from the star our observations are consistent with a gas-to-dust ratio of 100 but other values are not ruled out.