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

TL;DR

This study analyzes the carbon and oxygen isotopic abundances in the host star WASP-77A and its hot Jupiter to infer the planet's formation location relative to snowlines and its subsequent migration.

Contribution

It provides the first detailed isotopic abundance comparison between a host star and its exoplanet, revealing formation beyond snowlines and migration close to the star.

Findings

WASP-77A has a $^{12}$C/$^{13}$C ratio of 51±6, sub-solar but indicative of $^{13}$C enrichment.

WASP-77A b's $^{12}$C/$^{13}$C ratio is 26±16, suggesting formation beyond snowlines.

Chemical evidence supports the planet's migration to 0.024 AU from its star.

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 non-stellar 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 $51\pm 6$ for WASP-77A, which is sub-solar ($\sim 91$) but may still indicate $^{13}$C-enrichment in its companion planet WASP-77A b ($^{12}$C/$^{13}$C = 26 $\pm$ 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 $\sim$0.024 AU from its host star.