2024ApJ...974..151C

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 nonstellar 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 subsolar (∼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.