2024ApJ...970...55I

Recent high-resolution and sensitivity Atacama Large Millimeter/submillimeter Array observations have unveiled the carbon isotope ratios (¹²C/¹³C) of complex organic molecules (COMs) in a low-mass protostellar source. To understand the ¹²C/¹³C ratios of COMs, we investigated the carbon isotope fractionation of COMs from prestellar cores to protostellar cores with a gas-grain chemical network model. We confirmed that the ¹²C/¹³C ratios of small molecules are bimodal in the prestellar phase: CO and species formed from CO (e.g., CH₃OH) are slightly enriched in ¹³C compared to the local interstellar medium (by ∼10%), while those from C and C⁺ are depleted in ¹³C owing to isotope exchange reactions. COMs are mainly formed on the grain surface and in the hot gas (> 100 K) in the protostellar phase. The ¹²C/¹³C ratios of COMs depend on which molecules the COMs are formed from. In our base model, some COMs in the hot gas are depleted in ¹³C compared to the observations. Thus, we additionally incorporate reactions between gaseous atomic C and H₂O ice or CO ice on the grain surface to form H₂CO ice or C₂O ice, as suggested by recent laboratory studies. The direct C-atom addition reactions open pathways to form ¹³C-enriched COMs from atomic C and CO ice. We find that these direct C-atom addition reactions mitigate ¹³C-depletion of COMs, and the model with the direct C-atom addition reactions better reproduces the observations than our base model. We also discuss the impact of the cosmic-ray ionization rate on the ¹²C/¹³C ratio of COMs.