Metastable Helium Absorptions with 3D Hydrodynamics and Self-Consistent Photochemistry II: WASP-107b, Stellar Wind, Radiation Pressure, and Shear Instability

Abstract

This paper presents simulations of the metastable helium (He*) observations of WASP-107b, so far the highest signal-to-noise ratio detection that is confirmed by three different instruments. We employ full 3D hydrodynamics coupled with co-evolving non-equilibrium thermochemistry and ray-tracing radiation, predicting mass loss rates, temperature profiles, and synthetic He* line profiles and light curves from first principles. We found that a stellar wind stronger than solar is demanded by the observed heavily blueshifted line profile and asymmetric transit light curve. Contrary to previous beliefs, we argue that radiation pressure can be important for Ly$\alpha$ observations but {\it not} He*. We found WASP-107b is losing mass at a rate of $\dot{M} \simeq 1.0\times 10^{-9}~M_\oplus~{\rm yr}^{-1}$. Although $\dot{M}$ varies by $<1~\%$ given constant wind and irradiation from the host, shear instabilities still emerge from wind impacts, producing $\sim 10~\%$ fluctuations of He* transit depths over hour-long timescales. The common assumption that He* transit depth indicates the fluctuation of $\dot{M}$ is problematic. The trailing tail is more susceptible than planet adjacency to the shear instabilities, thus the line profile is more variable in the blue-shifted wing, while the transit light curve is more variable after mid-transit. We stress the synergy between Ly$\alpha$ (higher altitudes, lower density) and He* (lower altitudes, higher density) transit observations, particularly simultaneous ones, yield better understanding of planetary outflows and stellar wind properties.