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

TL;DR

This study investigates the distribution of gas and dust in the IM Lup protoplanetary disc, revealing a significant density drop near 400 AU and suggesting a high gas-to-dust ratio beyond this radius.

Contribution

It provides a detailed comparison of gas and dust distributions in the IM Lup disc, highlighting a density discontinuity and refining the disc model with observational data.

Findings

Gas disc extends to 900 AU, dust to 400 AU

Density drops sharply near 400 AU

Gas-to-dust ratio is at least 100 beyond 400 AU

Abstract

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. 12CO, 13CO, and C18O 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. The disc is seen at an inclination of 54+/-3 degrees and is in Keplerian rotation around a 0.8-1.6 Msun 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.d20 - 1.d22 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.