[P-CG22] Origin and evolution of materials in space

[PCG22-09] The impact of Cosmic Rays on Carbon Isotope Fractionation of Complex Organic Molecules in Star-Forming Cores

*Ryota Ichimura¹, Hideko Nomura¹, Kenji Furuya² (1.Division of Science, National Astronomical Observatory of Japan / Department of Astronomical Science The Graduate University for Advanced Studies, SOKENDAI , 2.Division of Science, National Astronomical Observatory of Japan)

Keywords:astrochemistry, star formation

Recent high-resolution and sensitivity ALMA 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 isotopic fractionation of COMs from prestellar cores to protostellar cores with a gas-grain chemical network model. 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 the molecules from which the COMs are formed and the reactions through which the COMs are formed. By incorporating 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, we find that these direct C-atom addition reactions mitigate carbon isotope fractionation. Also, the model with the direct C-atom addition reactions better reproduces the observations than the model without the direct C-atom addition reactions. However, CH₃OCH₃ in our results shows depletion in ¹³C compared to observations. We also investigate the effect of various cosmic ray (CR) ionization rates on the ¹²C/¹³C ratio of COMs. High CR ionization rates promoted ¹³C-depleted COMs via radical-radical reactions on warm grain surfaces during the collapse phase. For CH₃OCH₃, CR ionization contributes to its increase via ion-molecule reactions and subsequent dissociative recombination in the warm gas phase after water ice sublimation. Consequently, the ¹²C/¹³C ratio of CH₃OCH₃ decreases rapidly with time. When the CR ionization rates become 1.3×10⁻¹⁴ s⁻¹ only after protostar formation, this ratio becomes enriched in ¹³C. However, the ¹²C/¹³C ratio of CH₃OCH₃ even at the end of the collapse phase remains higher than the observations. We need more investigation to reproduce the observed carbon isotope fractionation of CH₃OCH₃.