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 H^12CN/H^13CN 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.