Isotopomer-Specific Carbon Isotope Ratio of Complex Organic Molecules in Star-Forming Cores

The recent observation of complex organic molecules (COMs) in interstellar ices by the James Webb Space Telescope (JWST), along with previous gas-phase detections, underscores the importance of grain surface and ice mantle chemistry in the synthesis of COMs. In this study, we investigate the formation and carbon isotope fractionation of COMs by constructing a new astrochemical reaction network that distinguishes the position of $^{13}$C within species (e.g., H$^{13}$COOCH$_3$ and HCOO$^{13}$CH$_3$ are distinguished). We take into account the position of $^{13}$C in each species in gas and solid phase chemistry. This new model allows us to resolve isotopomer-specific $^{12}$C/$^{13}$C ratios of COMs formed in the star-forming cores. We consider thermal diffusion-driven radical-radical reactions on the ice surface and non-thermal radiolysis chemistry in the bulk (surface + mantle) ice. We find that carbon isotope fractionation of the functional groups in COMs appears through both non-thermal radiolysis in cold environments and thermal diffusion in warm environments, depending on the COMs. In particular, COMs containing methyl groups show isotopomer differences in $^{12}$C/$^{13}$C ratios that reflect their formation pathways and environments. These isotopomer-resolved fractionation patterns provide a diagnostic tool to probe the origins of COMs in star-forming cores. Our results suggest that future comparisons between high-sensitivity isotopic observations and isotopomer-specific models will be helpful for constraining the relative contributions of thermal and non-thermal formation processes of COMs.

💡 Research Summary

This paper presents a novel astrochemical modeling study that investigates the origins of complex organic molecules (COMs) in star-forming cores by predicting position-specific carbon isotope ratios, known as isotopomer ratios. Motivated by recent detections of COMs in interstellar ices by JWST, the research addresses the challenge of distinguishing between different formation pathways (e.g., gas-phase, grain-surface thermal reactions, ice-mantle non-thermal processes) for the same observed molecule.

The core achievement is the development of a new, sophisticated chemical reaction network that moves beyond the traditional “full scrambling” assumption. Instead, it adopts a “position-conserved” approach for reactions on ice surfaces and mantles, explicitly tracking the location of a 13C atom within a molecule throughout reaction sequences. This allows the model to distinguish between isotopomers like H13COOCH3 and HCOO13CH3. The network, an extension of the Garroд (2013) framework, includes about 1600 new isotopically labeled species and 45,500 new reactions, incorporating both thermal diffusion-driven radical-radical reactions on ice surfaces and non-thermal radiolysis chemistry induced by cosmic rays within bulk ice.

Simulations are conducted for physical conditions evolving from a cold prestellar core to a warmer protostellar core. The results reveal that carbon isotope fractionation (deviation of 12C/13C from the local interstellar value of ~69) in COMs is highly position-specific. The fractionation patterns for different functional groups within a molecule vary significantly and are driven by different mechanisms depending on the environment: non-thermal radiolysis dominates in cold environments, while thermal diffusion reactions become important in warmer regions.

A key finding is that COMs containing methyl groups (e.g., acetaldehyde CH3CHO, dimethyl ether CH3OCH3) exhibit distinct isotopomer ratios between the methyl carbon and the carbon in other functional groups (e.g., the carbonyl carbon). This difference acts as a sensitive chemical fingerprint, encoding information about whether the molecule was formed primarily via thermal processes on warm grains or via non-thermal radiolysis in cold ice mantles. The study qualitatively compares its predictions with recent observations of CH3CHO isotopomers in the protostar V883 Ori, demonstrating the model’s potential utility.

In conclusion, the research establishes isotopomer-resolved carbon isotope ratios as a powerful new diagnostic tool for astrochemistry. By comparing future high-sensitivity observational data from facilities like ALMA and JWST with such position-specific models, astronomers will be better equipped to constrain the physical conditions and dominant chemical pathways responsible for COM formation in the diverse environments of star-forming regions, ultimately tracing the chemical heritage of organic matter from clouds to potential planetary systems.