Stellar wind interaction and pick-up ion escape of the Kepler-11 “super-Earths”

• Published in: Astronomy and astrophysics (Berlin) 2014-02, Vol.562, p.np-np
• Main Authors: Kislyakova, K. G., Johnstone, C. P., Odert, P., Erkaev, N. V., Lammer, H., Lüftinger, T., Holmström, M., Khodachenko, M. L., Güdel, M.
• Format: Article
• Language: English
• Subjects: Astronomy; Coronas; Escape systems; Estimates; Extrasolar planets; Gravitational effects; Hydrogen atoms; methods: numerical; planet-star interactions; planets and satellites: atmospheres; planets and satellites: individual: Kepler-11 system; Stellar winds
• ISSN: 0004-6361; 1432-0746
• DOI: 10.1051/0004-6361/201322933

Aims. 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. Methods. 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+ 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. Results. 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 ~6.4 × 1030 s-1 and ~4.1 × 1031 s-1, depending on the planet’s orbital location and assumed heating efficiency. These values correspond to non-thermal mass loss rates of ~1.07 × 107 g s-1 and ~6.8 × 107 g s-1 respectively, which is a few percent of the thermal escape rates.