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

Abstract The elemental and isotopic abundances of volatiles like carbon, oxygen, and nitrogen may trace a planet’s formation location relative to H 2 O, CO 2 , CO, NH 3 , and N 2 “snowlines,” or the distance from the star at which these volatile elements sublimate. By comparing the C/O and 12 C/ 13 C 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 nonstellar C/O and 13 C 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 12 C/ 13 C abundance analysis for host star WASP-77A (whose hot Jupiter’s 12 C/ 13 C 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 12 C/ 13 C ratio of 51 ± 6 for WASP-77A, which is subsolar (∼91) but may still indicate 13 C enrichment in its companion planet WASP-77A b ( 12 C/ 13 C = 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 H 2 O and CO 2 snowlines and provide chemical evidence for the planet’s migration to its current location ∼0.024 au from its host star.