2024ApJ...962L..30E

We present a comprehensive atmospheric retrieval study of the hot Jupiter WASP-77A b using eclipse observations from the Hubble Space Telescope (HST) and JWST. Using atmospheric retrievals, the spectral features of H₂O, CO, and TiO are identified, with volume mixing ratios estimated at log₁₀(VMR) = -4.40${}_{-0.11}^{+0.14}$, -4.44${}_{-0.28}^{+0.34}$, and -6.40${}_{-0.23}^{+0.22}$, respectively. We derive the atmospheric C-to-O ratio-a key planetary formation tracer-to be C/O = 0.54 ± 0.12, which is consistent with both the stellar host value and previous studies of the planet's atmosphere, suggesting a relatively close-in formation. Computing other elemental ratios (i.e., C/H, O/H, and Ti/H), we conclude that the general enrichment of the atmosphere (i.e., metallicity) is substellar, depleted in C and O, but that Ti appears slightly superstellar. A low C and O content could be obtained, in combination with a stellar C/O ratio if the planet formed outside of the CO₂ snow line before migrating inwards. Meanwhile, a superstellar Ti/H could be obtained by late contamination from refractory rich planetesimals. While broadly in agreement with previous works, we do find some differences and discuss these while also highlighting the need for homogeneous analyses when comparative exoplanetology is conducted.