A New Method for Direct Measurement of Isotopologue Ratios in Protoplanetary Disks: A Case Study of the 12CO/13CO Ratio in the TW Hya Disk

Planetary systems are thought to be born in protoplanetary disks. Isotope ratios are a powerful tool for investigating the material origin and evolution from molecular clouds to planetary systems via protoplanetary disks. However, it is challenging to measure the isotope (isotopologue) ratios, especially in protoplanetary disks, because the emission lines of major species are saturated. We developed a new method to overcome these challenges by using optically thin line wings induced by thermal broadening. As a first application of the method, we analyzed two carbon monoxide isotopologue lines, (CO)-C-12 3-2 and (CO)-C-13 3-2, from archival observations of a protoplanetary disk around TW Hya with the Atacama Large Millimeter/submillimeter Array. The (CO)-C-12/(CO)-C-13 ratio was estimated to be 21 +/- 5 at disk radii of 70-110 au, which is significantly smaller than the value observed in the local interstellar medium, similar to 69. It implies that an isotope exchange reaction occurs in a low-temperature environment with C/O > 1. In contrast, it is suggested that (CO)-C-12/(CO)-C-13 is higher than similar to 84 in the outer disk (r > 130 au), which can be explained by the difference in the binding energy of the isotopologues on dust grains and the CO gas depletion processes. Our results imply that the gas-phase (CO)-C-12/(CO)-C-13 can vary by a factor of >4 even inside a protoplanetary disk and therefore can be used to trace material evolution in disks.

Automated summary

Planetary systems are believed to originate from protoplanetary disks. Isotope ratios provide a powerful tool for studying the material origin and evolution in such systems. However, measuring isotope ratios, especially in protoplanetary disks, is challenging due to the saturation of emission lines. Researchers have developed a new method that utilizes optically thin line wings induced by thermal broadening to overcome these challenges. They applied this method to analyze carbon monoxide isotopologue lines in a protoplanetary disk and found variations in the (CO)-C-12/(CO)-C-13 ratio, suggesting material evolution within the disk.