2025ApJ...995..198B

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 multifrequency, multispecies, steady-state, hydrodynamic Parker wind photoevaporation relaxation model based on Murray-Clay et al. The model can move smoothly between high and low flux regimes and accepts custom multifrequency 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., 13.6-2000 eV) 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 ≈ 1.1R_P-1.8R_P for low fluxes, meaning that the energy-limited mass-loss rate, ${\dot{M}}_{{\rm{Elim}}}(R)$, 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 ${\dot{M}}_{{\rm{Elim}}}(R={R}_{{\rm{XUV}}})$ 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 $\dot{M}$.