A Survey of CH3CN and HC3N in Protoplanetary Disks

TL;DR

This survey investigates the presence and distribution of nitrile molecules CH3CN and HC3N in various protoplanetary disks, revealing consistent chemical patterns that influence planet and comet formation.

Contribution

It provides the first comprehensive observational analysis of CH3CN and HC3N in multiple protoplanetary disks, highlighting their abundance and temperature profiles.

Findings

HC3N detected in all but one disk, IM Lup.

CH3CN detected in three disks, indicating rich nitrile chemistry.

Nitrile abundances are similar to those in protostellar envelopes and comets.

Abstract

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 towards 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 towards all disks except IM Lup, and CH3CN is detected towards V4046 Sgr, MWC 480, and HD 163296. Rotational temperatures derived for disks with multiple detected lines range from 29-73K, 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.