The Wanderer: Charting WASP-77A b’s Formation and Migration Using a System-wide Inventory of Carbon and Oxygen Abundances

The elemental and isotopic abundances of volatiles like carbon, oxygen, and nitrogen may trace a planet's formation location relative to H₂O, CO₂, CO, NH₃, and N₂ "snowlines", or the distance from the star at which these volatile elements sublimate. By comparing the C/O and ^12C/^13C ratios measured in giant exoplanet atmospheres to complementary measurements of their host stars, we can determine whether the planet inherited stellar abundances from formation inside the volatile snowlines, or non-stellar C/O and ^13C enrichment characteristic of formation beyond the snowlines. To date, there are still only a handful of exoplanet systems where we can make a direct comparison of elemental and isotopic CNO abundances between an exoplanet and its host star. Here, we present a ^12C/^13C abundance analysis for host star WASP-77A (whose hot Jupiter's ^12C/^13C abundance was recently measured). We use MARCS stellar atmosphere models and the radiative transfer code TurboSpectrum to generate synthetic stellar spectra for isotopic abundance calculations. We find a ^12C/^13C ratio of 51 6 for WASP-77A, which is sub-solar (91) but may still indicate ^13C-enrichment in its companion planet WASP-77A b (^12C/^13C = 26 16, previously reported). Together with the inventory of carbon and oxygen abundances in both the host and companion planet, these chemical constraints point to WASP-77A b's formation beyond the H₂O and CO₂ snowlines and provide chemical evidence for the planet's migration to its current location 0. 024 AU from its host star.