Protoplanetary disk masses from CO isotopologue line emission

Context. One of the methods for deriving disk masses relies on direct observations of the gas, whose bulk mass is in the outer cold regions (T ≲ 30 K). This zone can be well traced by rotational lines of less abundant CO isotopologues such as 13CO, C18O, and C17O, which probe the gas down to the midplane. The total CO gas mass is then obtained with the isotopologue ratios taken to be constant at the elemental isotope values found in the local interstellar medium. This approach is imprecise, however, because isotope-selective processes are ignored.