Metastable Helium Absorptions with 3D Hydrodynamics and Self-Consistent Photochemistry I: WASP-69b, Dimensionality, XUV Flux Level, Spectral Types, and Flares

Abstract: The metastable Helium (He*) lines near $10830~{\rm A}$ are ideal probes of atmospheric erosion--a common phenomenon of close-in exoplanet evolution. A handful of exoplanet observations yielded well-resolved He* absorption features in transits, yet they were mostly analyzed with 1D isothermal models prescribing mass-loss rates. This work devises 3D hydrodynamics co-evolved with ray-tracing radiative transfer and non-equilibrium thermochemsitry. Starting from the observed stellar/planetary properties with reasonable assumptions about the host's high energy irradiation, we predict from first principle the mass loss rate, the temperature and ionization profiles, and 3D outflow kinematics. Our simulations well reproduce the observed He* line profiles and light curves of WASP-69b. We further investigate the dependence of He* observables on simulation conditions and host radiation. The key findings are: (1) Simulations reveal a photoevaporative outflow ($\sim 0.5~M_{\oplus}~{\rm Gyr}^{-1}$) for WASP-69b without a prominent comet-like tail, consistent with the symmetric transit shape (Vissapragada et al. 2020). (2) 3D simulations are mandatory for hydrodynamic features, including Coriolis force, advection, and kinematic line broadening. (3) EUV ($>13.6~{\rm eV}$) photons dominate photoevaporative outflows and populate He* via recombination; FUV is also detrimental by destroying He*; X-ray plays a secondary role. (4) K stars hit the sweet spot of EUV/FUV balance for He* line observation, while G and M stars are also worthy targets. (5) Stellar flars create characteristic responses in the He* line profiles.