HYDRODYNAMIC ESCAPE OF EXO-PLANETARY ATMOSPHERES

摘要

In recent studies of close-in giant exo-planets, the radiative effective temperature, which is not physically relevant for atmospheric escape processes, was used to estimate atmospheric evaporation rates (Konacki et al. 2003; Sasselov 2003). Therefore, these studies lead to significant underestimations of thermal atmospheric escape rates and to conclusions of long-term atmospheric stability. However, the temperature of the exosphere, which controls the thermal escape in an upper atmosphere, is usually much higher than the effective temperature, since upper planetary atmospheres are controlled by .absorption of X-rays and eXtreme UltraViolet (XUV) radiation (Bauer 1971). In this study, a scaling relation from solar system planets is used to estimate the exospheric temperature for giant exo-planets. This relation is based on the assumption of equilibrium between the XUV heat input and downward heat transport by conduction. We found that large exospheric temperatures, which are typical for hydrogen-dominated thermospheres, develop at close orbital distances to their host stars. These exosphere temperatures lead to an expansion, of the thermosphere and to hydrodynamic energy limited escape fluxes. Our atmospheric mass loss estimation applied to Jupiter-class exo-planets agree well with the recent H Lyman a detection of an extended exosphere around HD 209458b and its observation based estimated loss rate of about 10~(10) g s~(-1) (Vidal-Madjar et al. 2003).