Modeling the Effect of C/O Ratio on Complex Carbon Chemistry in Cold Molecular Clouds

TL;DR

This study explores how the C/O ratio influences complex carbon chemistry in cold molecular clouds, revealing high sensitivity of modeled abundances to this ratio and highlighting the importance of gas-phase and grain-surface processes.

Contribution

It systematically analyzes the impact of the C/O ratio on dark cloud chemistry using advanced modeling and machine learning, providing new insights into carbon reservoir dynamics.

Findings

Modeled abundances are highly sensitive to the C/O ratio.

CO and simple ices are major carbon reservoirs.

C3H4 isomers serve as significant reservoirs even in oxygen-rich conditions.

Abstract

Elemental abundances, which are often depleted with respect to the solar values, are important input parameters for kinetic models of interstellar chemistry. In particular, the amount of carbon relative to oxygen is known to have a strong effect on modeled abundances of many species. While previous studies have focused on comparison of modeled and observed abundances to constrain the C/O ratio, the effects of this parameter on the underlying chemistry have not been well-studied. We investigated the role of the C/O ratio on dark cloud chemistry using the NAUTILUS code and machine learning techniques for molecular representation. We find that modeled abundances are quite sensitive to the C/O ratio, especially for carbon-rich species such as carbon chains and polycyclic aromatic hydrocarbons (PAHs). CO and simple ice-phase species are found to be major carbon reservoirs under both oxygen-poor and oxygen-rich conditions. The appearance of C3H4 isomers as significant carbon reservoirs, even under oxygen-rich conditions, indicates the efficiency of gas-phase C3 formation followed by adsorption and grain-surface hydrogenation. Our model is not able to reproduce the observed, gas-phase C/H ratio of TMC-1 CP at the time of best fit with any C/O ratio between 0.1 and 3, suggesting that the modeled freeze-out of carbon-bearing molecules may be too rapid. Future investigations are needed to understand the reactivity of major carbon reservoirs and their conversion to complex organic molecules.