The First Detection of 13C17O in a Protoplanetary Disk: A Robust Tracer of Disk Gas Mass

Measurements of the gas mass are necessary to determine the planet formation potential of protoplanetary disks. Observations of rare CO isotopologues are typically used to determine disk gas masses; however, if the line emission is optically thick this will result in an underestimated disk mass. With the Atacama Large Millimeter/submillimeter Array we have detected the rarest stable CO isotopologue, (CO)-C-13-O-17, in a protoplanetary disk for the first time. We compare our observations with the existing detections of (CO)-C-12, (CO)-C-13, (CO)-O-18, and (CO)-O-17 in the HD 163296 disk. Radiative transfer modeling using a previously benchmarked model, and assuming interstellar isotopic abundances, significantly underestimates the integrated intensity of the (CO)-C-13-O-17 J = 3-2 line. Reconciliation between the observations and the model requires a global increase in CO gas mass by a factor of 3.5. This is a factor of 2-6 larger than previous gas mass estimates using (CO)-O-18. We find that (CO)-O-18 emission is optically thick within the snow line, while the (CO)-C-13-O-17 emission is optically thin and is thus a robust tracer of the bulk disk CO gas mass.