Mass loss from “Hot Jupiters”—Implications for CoRoT discoveries, Part II: Long time thermal atmospheric evaporation modeling

Abstract

We investigate the efficiency of the atmospheric mass loss due to hydrodynamic blow-off over the lifetime of the exoplanet HD209458b by studying numerically its hydrogen wind for host star X-ray and EUV (XUV) fluxes between 1 and 100 times that of the present Sun. We apply a time-dependent numerical algorithm which is able to solve the system of hydrodynamic equations straight through the transonic point of the flow including Roche lobe effects. The mass loss rates are calculated as functions of the absorbed energy in the thermosphere. Depending on the heating efficiency for a hydrogen-rich thermosphere the maximum temperature obtained in our study at 1.5 R pl by neglecting IR cooling is about 5000–10,000 K for heating efficiencies of 10% and 60%, respectively. We find that the upper atmosphere of HD209458b experiences hydrodynamic blow-off even at such low temperatures if one does not neglect gravitational effects caused by the proximity of the planet to its Roche lobe boundary. Depending on the heating efficiency, we find from the solution of the hydrodynamic equations of mass, momentum, and energy balance that energy-limited mass loss rate estimations overestimate the realistic mass loss rate at present time for HD209458b by several times. Using the maximum heating efficiency for hydrogen–rich atmospheres of 60% we find that HD209458b may experience an atmospheric mass loss rate at present time of about 3.5 × 10 10 g s - 1 . The mass loss rate evolves to higher values for higher XUV fluxes expected during the early period of the planet's host star evolution, reaching values of several times 10 12 g s - 1 . The integrated mass loss is found to be between 1.8% and 4.4% of the present mass of HD209458b. We found that the influence of the stellar tidal forces on atmospheric loss (the Roche lobe effect) is not significant at 0.045 AU. For a similar exoplanet, but at closer orbital distances ⩽ 0.02 AU , the combined effect of the Roche lobe and the high XUV radiation result in much higher thermal loss rates of about 2.6 × 10 11 g s - 1 and even more for early stages. This leads to a total loss over 4 Gyr of 27.5% of the planetary mass.