The role of C/O in nitrile astrochemistry in PDRs and planet-forming disks

TL;DR

This paper investigates how the carbon-to-oxygen ratio influences the formation of complex nitriles in photon-dominated regions and planet-forming disks, showing elevated C/O ratios can explain observed abundances.

Contribution

It demonstrates that gas-phase chemistry with high C/O ratios can account for nitrile observations in PDRs and disks, highlighting the importance of C/O in astrochemical modeling.

Findings

High C/O ratios (≥0.9) explain nitrile abundances in the Horsehead PDR.

Elevated C/O increases predicted nitrile abundances by several orders of magnitude.

C/O ratio is a key factor in modeling complex organic molecules in PDRs.

Abstract

Complex nitriles, such as HC3N, and CH3CN, are observed in a wide variety of astrophysical environments, including at relatively high abundances in photon-dominated regions (PDR) and the UV exposed atmospheres of planet-forming disks. The latter have been inferred to be oxygen-poor, suggesting that these observations may be explained by organic chemistry in C-rich environments. In this study we first explore if the PDR complex nitrile observations can be explained by gas-phase PDR chemistry alone if the elemental C/O ratio is elevated. In the case of the Horsehead PDR, we find that gas-phase chemistry with C/O $\gtrsim$ 0.9 can indeed explain the observed nitrile abundances, increasing predicted abundances by several orders of magnitude compared to standard C/O assumptions. We also find that the nitrile abundances are sensitive to the cosmic ray ionization treatment, and provide constraints on the branching ratios between CH3CN and CH3NC productions. In a fiducial disk model, an elevated C/O ratio increases the CH3CN and HC3N productions by more than an order of magnitude, bringing abundance predictions within an order of magnitude to what has been inferred from observations. The C/O ratio appears to be a key variable in predicting and interpreting complex organic molecule abundances in photon-dominated regions across a range of scales.