The impact of detailed alkali line profiles at extreme densities on giant planet interiors

Alkali lines, in particular the sodium (Na D) and potassium (K D) resonance doublets, are dominant opacity sources in giant planets over a wide range of temperatures (~ 1000 – 3000 K). Their strong pressure-broadened wings, produced by collisions with hydrogen, can significantly influence the thermal structure of giant planets, especially at high pressures. Most detailed line-profile calculations have so far been limited to moderate perturber densities up to 10²¹ cm^-3. However, conditions in the deep atmospheres and interiors of giant planets can reach significantly higher densities, causing temperature gradients to become increasingly uncertain. This is particularly relevant in the context of stable radiative layers that can exist in Solar System giants and warm giant exoplanets, as well as for hot giant planets that may remain radiative well beyond 100 bar.

In this work, we compute detailed Na D and K D line profiles using the unified line theory, extending to hydrogen perturber densities relevant for the deep atmospheres and interiors of giant planets. At high pressures, the revised line profiles exhibit significantly stronger and more extended wings than those predicted by the commonly used impact approximation, as well as density-dependent line shifts. We further show that Rosseland mean opacities can increase by up to an order of magnitude when using the revised line profiles. Consequently, the radiative-convective boundaries of warm and hot giant planets can shift to lower pressures, producing warmer adiabats and increasing inferred planetary bulk metallicities. While this work addresses the dominant absorption lines relevant for high-pressure opacity calculations in giant planets, it also highlights the broader need for accurate opacity modeling at extreme densities.