Wind-AE: A Fast, Open-source 1D Photoevaporation Code with Metal and Multi-frequency X-ray Capabilities

Abstract

Throughout their lives, short period exoplanets (<100 days) experience X-ray and extreme-UV (XUV) stellar irradiation that can heat and photoionize planets' upper atmospheres, driving transonic outflows. This photoevaporative mass loss plays a role in both evolution and observed demographics; however, mass loss rates are not currently directly observable and can only be inferred from models. To that end, we present an open-source fast 1D, XUV multi-frequency, multispecies, steady-state, hydrodynamic Parker Wind photoevaporation relaxation model based on Murray-Clay et al. (2009,arXiv:0811.0006). The model can move smoothly between high and low flux regimes and accepts custom multi-frequency stellar spectra. While the inclusion of high-energy X-rays increases mass loss rates ($\dot{M}$), metals decrease $\dot{M}$, and the net result for a typical hot Jupiter is a similar $\dot{M}$, but a hotter, faster, and more gradually ionized wind. We find that mulitfrequency photons (e.g., 13.6-2000eV) are absorbed over a broader range of heights in the atmosphere resulting in a wind-launch radius, $R_{XUV}$, that is of order 10 nanobars for all but the highest surface gravity planets. Grids of H/He solar metallicity atmospheres reveal that, for typical hot Jupiters like HD 209458b, $R_{XUV}$~1.1-1.8$R_P$ for low-fluxes, meaning that the energy-limited mass loss rate, $\dot{M}_{Elim}(R)$, computed at $R=R_P$ is a good approximation. However, for planets with low escape velocities, like many sub-Neptunes and super-Earths, $R_{XUV}$ can be >>$R_P$, making it necessary to use $\dot{M}_{Elim}(R=R_{XUV})$ to avoid significantly underestimating mass loss rates. For both high escape velocities and large incident fluxes, radiative cooling is significant and energy-limited mass loss overestimates $\dot{M}$.