Non-LTE Models of (Cool) Stellar and Giant Planetary Atmospheres

Abstract

The atmospheres of cool stars, brown dwarfs and extrasolar giant planets are rich with a large mix of molecules producing a complicated spectrum dominated by molecular absorption bands and, consequently, highly non‐Planckian shape. Additionally, many discovered extrasolar planets and brown dwarfs are close to a nearby, much hotter, main sequence star. Either of the above two conditions cause large deviations from LTE if the collisional processes are insufficient to restore LTE in the atmospheric layers.Here we review the general conditions which drive stellar atmospheres in non thermal equilibrium (Non‐LTE). Furthermore, we present recent advances in simulating non‐LTE effects in cool atmospheres for both non‐irradiated and irradiated stars and planets. In non‐irradiated atmospheres, the radiative rates are often too small compared to the collisional rates to drive large deviations from LTE. However, irradiated atmospheres show significant non‐LTE effects, e.g., in the recently discovered planet HD209458b eclipsing its parent star.