Carbon Isotope Chemistry in Protoplanetary Disks: Effects of C/O Ratios

Carbon isotope fractionation of CO has been reported in the disk around TW Hya, where elemental carbon is more abundant than elemental oxygen ([C/O]_elem > 1). We investigated the effects of the [C/O]_elem ratio on carbon fractionation using astrochemical models that incorporate isotope-selective photodissociation and isotope exchange reactions. The ¹²CO/¹³CO ratio could be lower than the elemental carbon isotope ratio due to isotope exchange reactions when the [C/O]_elem ratio exceeds unity. The observed ¹²CO/¹³CO and H¹²CN/H¹³CN ratios around TW Hya could be reproduced when the [C/O]_elem ratio is 2–5. In the vicinity of the lower boundary of the warm molecular layer, the formation of ices leads to the gas-phase [C/O]_elem ratio approaching unity, irrespective of the total (gas + ice) [C/O]_elem ratio. This phenomenon reduces the variation in the ¹²CO/¹³CO ratio across different [C/O]_elem ratios.