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

TL;DR

This work addresses how the carbon-to-oxygen ratio influences complex carbon chemistry in cold molecular clouds, a key driver of molecular inventories in star-forming regions. Using the NAUTILUS 3-phase chemical model and a VICGAE/SELFIES/UMAP embedding to visualize the network, the authors explore 0.1$\le$C/O$\le$3.0 and identify regime-dependent sensitivities, with carbon-chain and PAH-related chemistry becoming more pronounced as C/O rises. They find CO and simple ice-phase species remain major carbon reservoirs across conditions, while C/O>1 markedly enhances unsaturated carbon-chain formation and ice-mantle hydrocarbons, though the model struggles to reproduce the observed gas-phase C/H ratio in TMC-1 CP, suggesting faster-than-realistic freeze-out or missing desorption/carbon-sourcing processes. The study highlights key pathways and reservoirs that shape interstellar carbon chemistry and provides a framework for linking cloud-phase chemistry to protoplanetary disk evolution, while underscoring the need for improved desorption mechanisms and fuller treatment of C3H$_n$ and PAHs. These insights have implications for interpreting molecular inventories in dense clouds and for informing disk chemistry and planet-forming environments.

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.

Figures (10)

• Figure 1: Top 10 most sensitive carbon-containing species to the C/O ratio at $5\times10^5$ years according to relative standard deviation. Relative standard deviations are shown for three different ranges of C/O ratio as discussed in the text: $\rm{C/O} = 0.1 - 1.0$ (orange), $\rm{C/O} = 0.5 - 2.0$ (green), and $\rm{C/O} = 2.0 - 3.0$ (blue). Each bar is annotated with the formula of the corresponding species as it appears in the model. Species names with a prefix of "s-" correspond to ice-phase species where the abundances in the surface and mantle phases have been summed together
• Figure 2: Modeled abundances relative to H of C11 as a function of time for every tested C/O ratio. The minimum ratio of 0.1 is shown in red, the nominal ratio of 1.1 is shown in orange, and the maximum ratio of 3.0 is shown in green.
• Figure 3: Same as Figure \ref{['fig:C11']} but for C10H8.
• Figure 4: Modeled time-dependent carbon fractions of every carbon-containing species in the network with a C/O ratio of 0.5. Each dot represents a species that contains carbon plotted according to its UMAP vectors. The color corresponds to the elemental composition, while the size is the fraction of carbon multiplied by a scaling factor. Molecular structures are shown for species comprising at least 2% of the total carbon. Some of structures are slightly offset from their corresponding dots for visual clarity. Structures for ice-phase (grain surface or mantle) species are highlighted in green. At the beginning of the simulation, nearly all of the carbon is in atomic form as represented by a large black dot in the center-right of the figure. Starting at $\sim100$ years, a red dot slightly to the left of atomic carbon corresponding to CO begins to grow. At $500$ years, a handful of dots clustered near the upper right corner also begin to slowly grow. These correspond to ice-phase HCN, H2CO, CH4, and CH3NH2. These species continue to grow in carbon fraction as time progresses, with CO reaching the 2% of total carbon threshold at 4695 years. Starting at $10^4$ years, the dot corresponding to atomic carbon begins to rapidly diminish while the aforementioned dots grow more rapidly. A black dot below CO corresponding to C3 quickly grows from this point, reaching the 2% threshold at 11220 years. At 19420 years ice-phase CO also reaches the 2% threshold. At $3\times10^4$ years ice-phase H2CO reaches 2% of the total carbon budget, followed shortly by HCN, CH3NH2, and CH4 all in the ice phase as well. At $10^5$ years atomic carbon begins to rapidly shrink in abundance, followed shortly by C3. Gas-phase CO peaks in size at $\sim2\times10^5$ years and then decreases whereas ice-phase CO rapidly expands. There are also a few dots representing pure hydrocarbons, oxygen-containing species, and nitrogen-containing species that become sizable in terms of carbon fraction. The only one of these that reaches the 2% threshold is ice-phase CH2CCH2 at $3.54\times10^5$, located a bit to the left of CO and growing in size until $\sim10^6$ years. Between $10^6$ years and the end of the simulation at $10^7$ years there are few changes except a decline in CH3NH2, the appearance of ice-phase HNC as significant carbon reservoir near CO and atomic carbon, and continued growth of ice-phase CH4. The static image displays the major reservoirs of carbon (those containing at least 2% of the total carbon) at $\sim5\times10^5$ years, where our model best reproduces the abundances of larger carbon-bearing molecules.
• Figure 5: Same as Figure \ref{['fig:C-poor-Cfrac']} but with a C/O ratio of 1. The evolution of carbon fractions begins very similarly to that of a C/O ratio of 0.5, although CO forms more slowly and does not reach the 2% threshold until 5698 years. C3 reaches this same threshold immediately after at a time of 5791 years. As in Figure \ref{['fig:C-poor-Cfrac']}, atomic carbon begins to quickly diminish at $10^4$ years whereas C3, CO, ice-phase CO, and ice-phase HCN, H2CO, CH4, and CH3NH2 grow in size more quickly. Ice-phase CO achieves a carbon fraction of 2% at 26390 years, followed by HCN, H2CO, CH4, and CH3NH2 at $\sim4\times10^4$ years. By $10^5$ years C3 is the second largest carbon reservoir next to CO, almost triple the size as in the model with a C/O of 0.5 at this time. Following this time there is rapid gain of ice-phase CO and loss of atomic carbon and C3 followed by gas-phase CO, although C3 maintains a carbon fraction of at least 2% until a time of $3.78\times10^5$ years compared to $3.02\times10^5$ years in Figure \ref{['fig:C-poor-Cfrac']}. A growth in various nitrogen-containing, oxygen-containing, and pure hydrocarbon species is again seen although at an earlier time and to a stronger extent. CH2CCH2 reaches the 2% threshold at $1.58\times10^5$ years followed by its isomer CH3CCH at $1.83\times10^5$ years. The dot for CH3CCH is located a good distance below that of CH2CCH2 at the top of a series of dots that extends down to the bottom of the graph. Between $10^5$ and $\sim5\times10^5$ many dots in this series, along with another series on the right side of the figure below CO, grow in size sequentially, although none of them reach a carbon fraction of 2%. The carbon fraction of CH3CCH diminishes from $3\times10^3$ years and drops below 2% by $4.37\times10^5$ years, while the fraction of carbon in CH2CCH2 continues to grow. Between $10^5$ years there is also a decline in ice-phase CH3NH2 and a growth in ice-phase CH4. From $10^6$ to $10^7$ years the major observations are the appearance of ice-phase C3H8 as a major carbon reservoir at $10^6$ years in the top-right cluster, a continued growth in ice-phase CH4, and the continued diminishing of various other species such as the series of dots that extend toward the bottom of the figure.