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]${}_{\mathrm{e}\mathrm{l}\mathrm{e}\mathrm{m}}$> 1). We investigated the effects of the [C/O]${}_{\mathrm{e}\mathrm{l}\mathrm{e}\mathrm{m}}$ 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]${}_{\mathrm{e}\mathrm{l}\mathrm{e}\mathrm{m}}$ 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]${}_{\mathrm{e}\mathrm{l}\mathrm{e}\mathrm{m}}$ 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]${}_{\mathrm{e}\mathrm{l}\mathrm{e}\mathrm{m}}$ ratio approaching unity, irrespective of the total (gas + ice) [C/O]${}_{\mathrm{e}\mathrm{l}\mathrm{e}\mathrm{m}}$ ratio. This phenomenon reduces the variation in the ${}^{12}$CO/${}^{13}$CO ratio across different [C/O]${}_{\mathrm{e}\mathrm{l}\mathrm{e}\mathrm{m}}$ ratios.