Benchmarking 13C fractionation with HC3N in protoplanetary disks

Ryan Alexander Loomis

Isotope ratios (e.g. D/H, 12C/13C, 14N/15N) are powerful tools in the study of bodies within the Solar System, shedding light on questions such as the origin of the Earth's water reservoir. Connect these isotopic measurements to protoplanetary disks has proven difficult, however, with isotope fractionation studies limited by an unconstrained 12C/13C ratio. An ISM ratio of 69 is often assumed, but disk chemical modeling has shown that this value can be modified by factors of >5 by in-situ chemistry. Typical bright tracers (e.g. CO, CS, CN) are too optically thick in their 12C isotopologues to make a direct measurement, and weaker species (e.g. CH3CN) do not have strong enough 13C isotopologue emission.

Here we present a direct measurement of 12C/13C using HC3N observations toward the nearest disk TW Hya. HC3N is bright enough to allow for detection of its isotopologues, but not so optically thick in the main species as to prevent constraint of its column density. We show that the disk 12C/13C ratio is substantially modified from the canonical ISM value, and demonstrate differential fractionation for the 13CN bearing isotopologue.