Abstract
<p>The mass of an exoplanet is its most fundamental property. Currently, masses are typically determined via the radial velocity (RV) technique (since the transit-timing variation method requires special planetary systems); unfortunately, sufficiently precise RVs cannot be acquired for a significant fraction of planet-hosting stars. Transmission spectroscopy has also been proposed as a way to derive masses for transiting planets, by combining the radius inferred from transits with the surface gravity derived from the Rayleigh scattering slope in the spectrum. However, this only works for planets with known atmospheric metallicity (e.g., solar), since the Rayleigh slope has a gravity-metallicity degeneracy.  Using a simple analytical model, we show here that: (1) this degeneracy can be broken for H/He-dominated atmospheres, and thus mass and metallicity derived independently from transmission spectra, by simultaneous consideration of the Rayleigh scattering slope and the pressure-broadened Sodium line profile; and (2) with ground-based high-resolution spectra (e.g., from VLT-FORS2) of currently achieved S/N, the derived masses are sufficiently accurate to distinguish between gas giants and sub-Neptunes (which is vital for constraining planet formation and evolution theories).  We validate these statements by comparing our models to: (a) the observed spectrum of the transiting gas giant WASP 96b (M<sub>P</sub> ~ 144 M<sub>Earth</sub>  from RV, R<sub>P</sub> ~ 1.2 R<sub>Jup</sub>), and (b) a model spectrum of the transiting young sub-Neptune V1298 Tau c (M<sub>P</sub> < 28 M<sub>Earth</sub> from dynamics, R<sub>P</sub> ~ 0.5 R<sub>Jup</sub>). In both cases we recover the known (or dynamically constrained) mass, and do so precisely enough to clearly distinguish between the two (despite their similar radii).  Finally, the Potassium feature is substantially weaker than the Sodium one in the observed spectra of a number of close-in planets (e.g., WASP 96b); this has previously been ascribed to anomalously low Potassium abundance. Instead, our models show that (3) this asymmetry between Potassium and Sodium is a natural consequence of the vertical temperature profile that results if heat is redistributed between the day and night sides of tidally-locked planets (as expected due to, e.g., winds). Thus, weak Potassium can be used as a signature of such redistribution.</p>
Published Version
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