2018ApJ...857...69B

The organic content of protoplanetary disks sets the initial compositions of planets and comets, thereby influencing subsequent chemistry that is possible in nascent planetary systems. We present observations of the complex nitrile-bearing species CH₃CN and HC₃N toward the disks around the T Tauri stars AS 209, IM Lup, LkCa 15, and V4046 Sgr as well as the Herbig Ae stars MWC 480 and HD 163296. HC₃N is detected toward all disks except IM Lup, and CH₃CN is detected toward V4046 Sgr, MWC 480, and HD 163296. Rotational temperatures derived for disks with multiple detected lines range from 29 to 73 K, indicating emission from the temperate molecular layer of the disk. V4046 Sgr and MWC 480 radial abundance profiles are constrained using a parametric model; the gas-phase CH₃CN and HC₃N abundances with respect to HCN are a few to tens of percent in the inner 100 au of the disk, signifying a rich nitrile chemistry at planet- and comet-forming disk radii. We find consistent relative abundances of CH₃CN, HC₃N, and HCN between our disk sample, protostellar envelopes, and solar system comets; this is suggestive of a robust nitrile chemistry with similar outcomes under a wide range of physical conditions.