The Distribution and Excitation of CH 3 CN in a Solar Nebula Analog

Abstract

Cometary studies suggest that the organic composition of the early Solar Nebula was rich in complex nitrile species such CH 3 CN. Recent ALMA detections in protoplanetary disks suggest that these species may be common during planet and comet formation, but connecting gas-phase measurements to cometary abundances first requires constraints on formation chemistry and distributions of these species. We present here the detection of seven spatially resolved transitions of CH 3 CN in the protoplanetary disk around the T-Tauri star TW Hya. Using a rotational diagram analysis, we find a disk-averaged column density of cm −2 and a rotational temperature of K. A radially resolved rotational diagram shows the rotational temperature to be constant across the disk, suggesting that the CH 3 CN emission originates from a layer at z / r  ∼ 0.3. Through comparison of the observations with predictions from a disk chemistry model, we find that grain-surface reactions likely dominate CH 3 CN formation and that in situ disk chemistry is sufficient to explain the observed CH 3 CN column density profile without invoking inheritance from the protostellar phase. However, the same model fails to reproduce a solar system cometary abundance of CH 3 CN relative to H 2 O in the midplane, suggesting that either vigorous vertical mixing or some degree of inheritance from interstellar ices occurred in the Solar Nebula.