Transient Sodium and Potassium Clouds at Candidate Exomoon Systems

We present time-dependent simulations to study evaporative transmission spectra at candidate exo-Io systems. Several close-in gas giant systems are now known to possess minimum column densities, of neutral sodium and potassium, 〈N〉_min ≥ ~10¹⁰ – 10¹¹ cm^-2, that are able to be sustained in the planetary exosphere via the evaporation of rocky exomoons or exoring grains, during transit. However, the proximity of the stellar tide, along with the photoionization lifetime of τ_Na < 5 minutes within an orbit <10 R_Sun, strongly suggest any Jupiter-Io interaction should display time-dependent behavior in individual exposures associated with the close-in orbit of a putative third body. Time-series data from 2011-2021 of WASP-49, a G6V star (K = 9.75) taken by the high-resolution spectrographs: ESPRESSO/VLT, Keck/HIRES, and HARPS/ESO-3.6-m, are analyzed before, during, and after the transit of WASP-49 b, a hot Saturn orbiting at 2.8 days. We report a transient sodium signature occulting 3.53% of NaD₂ light at 3.1σ confidence, far larger than 〈N〉_min , during the transit of the gas giant’s Hill sphere ~ 100 minutes before transit begins. Transient and sustained alkali line absorption depths are systematically compared with several other exo-Io candidate systems to test the validity of the interpretations, as D2/D1 and Na/K ratios are known to characterize evaporative rock and ice processes in the solar system. We discuss conundrums with an exomoon-cloud scenario, and how alkali signatures at certain exoplanets may not be planetary, as recently suggested at HD-209458 b. Future infrared observatories may identify progenitor molecules of the evaporating atoms we observe here (e.g. dusty exorings/exomoons), as well as the thermal emission of tidally-heated molten rock.