Carbon Isotope Fractionation of Complex Organic Molecules in Star-Forming Cores

TL;DR

This study models carbon isotope fractionation in complex organic molecules during star formation, revealing how formation pathways and reactions influence isotope ratios and aligning models with recent ALMA observations.

Contribution

It introduces a chemical network model including direct C-atom addition reactions that better explains observed isotope ratios in star-forming regions.

Findings

COMs show bimodal $^{12}$C/$^{13}$C ratios in prestellar phases.

Direct C-atom addition reactions reduce $^{13}$C depletion in COMs.

Model with added reactions aligns better with ALMA observations.

Abstract

Recent high-resolution and sensitivity ALMA observations have unveiled the carbon isotope ratios ($^{12}$C/$^{13}$C) of Complex Organic Molecules (COMs) in a low-mass protostellar source. To understand the $^{12}$C/$^{13}$C 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 $^{12}$C/$^{13}$C ratios of small molecules are bimodal in the prestellar phase: CO and species formed from CO (e.g., CH$_{3}$OH) are slightly enriched in $^{13}$C compared to the local ISM (by $\sim$ 10 $\%$), while those from C and C$^{+}$ are depleted in $^{13}$C 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 $^{12}$C/$^{13}$C 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 $^{13}$C compared to the observations. Thus, We additionally incorporate reactions between gaseous atomic C and H$_{2}$O ice or CO ice on the grain surface to form H$_2$CO ice or \ce{C2O} ice, as suggested by recent laboratory studies. The direct C-atom addition reactions open pathways to form \ce{^13C}-enriched COMs from atomic C and CO ice. We find that these direct C-atom addition reactions mitigate $^{13}$C-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 $^{12}$C/$^{13}$C ratio of COMs.