2018ApJ...859..131L

Cometary studies suggest that the organic composition of the early Solar Nebula was rich in complex nitrile species such CH₃CN. 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 CH₃CN in the protoplanetary disk around the T-Tauri star TW Hya. Using a rotational diagram analysis, we find a disk-averaged column density of N_T=1.45_–0.15^+0.19×10¹² cm^–2 and a rotational temperature of T_rot=32.7_–3.4^+3.9 K. A radially resolved rotational diagram shows the rotational temperature to be constant across the disk, suggesting that the CH₃CN 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 CH₃CN formation and that in situ disk chemistry is sufficient to explain the observed CH₃CN column density profile without invoking inheritance from the protostellar phase. However, the same model fails to reproduce a solar system cometary abundance of CH₃CN relative to H₂O in the midplane, suggesting that either vigorous vertical mixing or some degree of inheritance from interstellar ices occurred in the Solar Nebula.