Sodium Evidence of a Rocky Exomoon

Neutral sodium is a geological beacon in our solar system, representative of rocky meteors in the Earth's atmosphere, regolith sputtering and desorption at the Moon and Mercury, volcanic outgassing at Jupiter's moon Io, and saltwater ascent at Enceladus and Europa. Due to the favorable cross section of sodium (NaD) and potassium (KD) to probe tenuous gases, indirect inferences of active, rocky exomoons can be made in the optical by carefully monitoring Doppler shifts at common alkali exoplanet systems. Notably, we present new Keck Planet Finder (KPF) observations of WASP-49 A b characterizing the sodium exosphere of WASP-49 A b, a hot Saturn exoplanet, and its putative satellite WASP-49 A b I. Based on new KECK observations, we present 3-D numerical gas tori and cloud models of the orbiting sodium with SERPENS, comparing also to candidate exomoon systems HD-189733 b II and WASP-39. The indirect sodium (and volcanic gas) spectroscopy method described here is directly applicable to the search for active exoplanets in general as, similar to Jupiter-Io, volcanic and sputtered sodium gas is readily ionized undergoing a series of plasma processes affecting the lifetime and morphology of neutral sodium.  Previous HARPS/3.6-m and KECK/HIRES observations suggest an exomoon orbit near the Roche limit at ~1.2 Rp, which is also consistent with recent dynamical studies. However, dozens more observations are needed to build a sodium radial velocity curve, to constrain an exomoon orbital period with reasonable accuracy, should the cloud indeed be bound to the exomoon.