Alkali lines at extreme densities and their impact on giant planet interior structure

Context . Alkali lines, in particular the sodium Na D (5891 Å, 5897 Å) and potassium K D (7667 Å, 7701 Å) resonance doublets, are dominant opacity sources in giant planets over a wide range of temperatures (≳1000 K). Their strong pressure-broadened wings significantly influence the thermal structure of giant planets, especially at high pressures. Most detailed line-profile calculations have so far been limited to perturber densities up to 10 21 cm −3 . However, conditions in the deep atmospheres and interiors of giant planets can reach significantly higher densities, making the temperature gradients increasingly uncertain. Aims . We determined how physically consistent collisional broadening of the Na D and K D lines at extreme densities affects opacity calculations and consequently the inferred interior structure of giant planets. Methods . We computed detailed Na D and K D line profiles using unified line theory, extending to molecular hydrogen perturber densities of n H 2 = 5×10 22 cm −3 , which translates to pressures up to ∼20 kbar. The revised cross sections were incorporated into Rosseland mean opacity tables, which were then used to evaluate their effect on planetary thermal structures. Results . At densities n H 2 > 10 21 cm −3 , the line profiles predicted by unified line theory exhibit significantly stronger and more extended wings than commonly used Voigt profiles, as well as density-dependent line shifts. The revised line profiles increase Rosseland mean opacities by a factor of 2 at 10 3 bar and by an order of magnitude at 10 4 bar. Consequently, the radiative-convective boundary of warm and hot giant planets can shift to lower pressures, producing warmer interior adiabats and increasing inferred core masses by up to 9 M earth . We further find that Jupiter is highly unlikely to host a stable radiative layer at the present time or throughout most of its evolution, as the required Na and K abundances for this are well below observational constraints. Our new opacity tables span metal mass fractions Z ∈ [0.0044, 0.696] and are publicly available.