A break in the gas and dust surface density of the disc around the T Tauri star IM Lupi

Aims. We study the distribution and physical properties of molecular gas in the disc around the T Tauri star IM Lup on scales close to 200 AU. 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. Methods. (CO)-C-12, (CO)-C-13, and (CO)-O-18 J = 2-1 line emission, as well as the dust continuum at 1.3 mm, 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. Results. The disc is seen at an inclination of 54 degrees +/- 3 degrees and is in Keplerian rotation around a 0.8-1.6 M-circle dot star. The outer disc radius traced by molecular gas emission is 900 AU, while the dust continuum emission and scattered light images limit the amount of dust present beyond 400 AU and are consistent with the existing model that assumes a 400 AU radius. Our observations require a drastic density decrease close to 400 AU with the vertical gas column density at 900 AU in the range of 5 x 10(20)-10(22) cm(-2). We derive a gas-to-dust mass ratio of 100 or higher in disc regions beyond 400 AU. Within 400 AU from the star our observations are consistent with a gas-to-dust ratio of 100 but other values are not ruled out.