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 12 CO/ 13 CO 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 12 CO/ 13 CO and H 12 CN/H 13 CN 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 12 CO/ 13 CO ratio across different [C/O] elem ratios.