Blowin' in the non-isothermal wind: core-powered mass loss with hydrodynamic radiative transfer

The mass loss rates of planets undergoing core-powered escape are usually modeled using an isothermal Parker-type wind at the equilibrium temperature, Tₑq. However, the upper atmospheres of sub-Neptunes may not be isothermal if there are significant differences between the opacity to incident visible and outgoing infrared radiation. We model bolometrically-driven escape using aiolos, a hydrodynamic radiative-transfer code that incorporates double-gray opacities, to investigate the process's dependence on the visible-to-infrared opacity ratio,. For a value of 1, we find that the resulting mass loss rates are well-approximated by a Parker-type wind with an isothermal temperature T = Tₑq/2^1/4. However, we show that over a range of physically plausible values of, the mass loss rates can vary by orders of magnitude, ranging from 10^-5 the isothermal rate for low to 10⁵ the isothermal rate for high. The differences in mass loss rates are largest for small planet radii, while for large planet radii, mass loss rates become nearly independent of and approach the isothermal approximation. We incorporate these opacity-dependent mass loss rates into a self-consistent planetary mass and energy evolution model and show that lower/higher values lead to more/less hydrogen being retained after core-powered mass loss. In some cases, the choice of opacities determines whether or not a planet can retain a significant primordial hydrogen atmosphere. The dependence of escape rate on the opacity ratio may allow atmospheric escape observations to directly constrain a planet's opacities and therefore its atmospheric composition.