2014A&A...562A.116K

We study the interactions between stellar winds and the extended hydrogen-dominated upper atmospheres of planets. We estimate the resulting escape of planetary pick-up ions from the five ``super-Earths'' in the compact Kepler-11 system and compare the escape rates with the efficiency of the thermal escape of neutral hydrogen atoms. Assuming the stellar wind of Kepler-11 is similar to the solar wind, we use a polytropic 1D hydrodynamic wind model to estimate the wind properties at the planetary orbits. We apply a direct simulation Monte Carlo model to model the hydrogen coronae and the stellar wind plasma interaction around Kepler-11b-f within a realistic expected heating efficiency range of 15-40%. The same model is used to estimate the ion pick-up escape from the XUV heated and hydrodynamically extended upper atmospheres of Kepler-11b-f. From the interaction model, we study the influence of possible magnetic moments, calculate the charge exchange and photoionization production rates of planetary ions, and estimate the loss rates of pick-up H<sup>+</sup> ions for all five planets. We compare the results between the five ``super-Earths'' and the thermal escape rates of the neutral planetary hydrogen atoms. Our results show that a huge neutral hydrogen corona is formed around the planet for all Kepler-11b-f exoplanets. The non-symmetric form of the corona changes from planet to planet and is defined mostly by radiation pressure and gravitational effects. Non-thermal escape rates of pick-up ionized hydrogen atoms for Kepler-11 ``super-Earths'' vary between ∼6.4x10³⁰/s and ∼4.1x10³¹/s, depending on the planet's orbital location and assumed heating efficiency. These values correspond to non-thermal mass loss rates of ∼1.07x10⁷g/s and ∼6.8x10⁷g/s respectively, which is a few percent of the thermal escape rates.