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 ALMA we
have detected the rarest stable CO isotopologue, 13C17O, in a protoplanetary
disk for the first time. We compare our observations with the existing
detections of 12CO, 13CO, C18O and C17O in the HD163296 disk. Radiative
transfer modelling using a previously benchmarked model, and assuming
interstellar isotopic abundances, significantly underestimates the integrated
intensity of the 13C17O 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 C18O. We find
that C18O emission is optically thick within the CO snow line, while the 13C17O
emission is optically thin and is thus a robust tracer of the bulk disk CO gas
mass.
