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

Résumé fourni par la source

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 ($\gtrsim 1000 \ \rm K$). 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 $10^{21} \ \rm cm^{-3}$. However, conditions in the deep atmospheres and interiors of giant planets can reach significantly higher densities, making the temperature gradients increasingly uncertain. We determine 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. We compute detailed Na $D$ and K $D$ line profiles using the unified line theory, extending to molecular hydrogen perturber densities of $n_{\rm H_2} = 5 \times 10^{22} \ \rm cm^{-3}$, which translates to pressures up to $\sim 20 \ \rm kbar$. The revised cross-sections were incorporated into Rosseland mean opacity tables, which were then used to evaluate their effect on planetary thermal structures. At densities $n_{\rm H_2} > 10^{21} \ \rm cm^{-3}$, the line profiles predicted by the 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 $10^3 \ \rm bar$ and by an order of magnitude at $10^4 \ \rm bar$. 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 $9 \ \rm M_{\rm earth}$.

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