MOPSS II: Extreme Optical Scattering Slope for the Inflated Super-Neptune HATS-8b

We present results for the inflated super-Neptune HATS-8b from MOPSS, The Michigan Optical Planetary Spectra Survey. This program is aimed at creating a database of optical planetary transmission spectra all observed, reduced, and analyzed with a uniform method for the benefit of enabling comparative exoplanet studies. HATS-8b orbits a G dwarf and is a low density super-Neptune, with a radius of 0.873 R$_{Jup}$, a mass of 0.138 M$_{Jup}$, and a density of 0.259 g/cm$^3$. Two transits of HATS-8b were observed in July and August of 2017 with the IMACS instrument on the Magellan Baade 6.5m telescope. We find an enhanced scattering slope that differs between our two nights. These slopes are stronger than one due only to Rayleigh scattering and cannot be fully explained by unocculted star spots. We explore the impact of condensates on the scattering slope and determine that MnS particulates smaller than 10$^{-2}\mu$m can explain up to 80\% of our measured slope if the planet is warmer than equilibrium, or 50\% of the slope at the equilibrium temperature of the planet for a low mean molecular weight atmosphere. The scattering slope that we observe is thus beyond even the most extreme case predicted by theory. We suggest further follow up on this target and host star to determine if the temporal variation of the slope is primarily due to stellar or planetary effects, and to better understand what these effects may be.