Abstract

1. Stellar wind interaction modelling of exoplanetupper atmospheres K.G. Kislyakova1, H. Lammer1, M. Holmström2, P. Odert3, M. L. Khodachenko1 1IWF/ÖAW, Graz/Austria, 2IRF, Kiruna/Sweden, 3IGAM/KFUG, Graz/Austria Abstract Ly- transit study of magnetospheres We present a numerical method for modeling of the stellar wind interaction and the formation of hydrogen coronae around exoplanets. The method allows multispecies processing. At present it includes neutral hydrogen atoms and hydrogen ions and is based on a Direct Simulation Monte Carlo (DSMC) algorithm (Holmström et al., Nature, 2008). In combination with a hydrodynamic code (Erkaev et al., Astob. 13, 1011, 2013), it allows us to estimate the thermal and non-thermal escape rates from hydrogen-dominated upper atmospheres, including the study on the atmospheric evolution. In combination with observations in Ly-, the algorithm can also be used to estimate the planetary magnetic field strength and various plasma environment properties in the vicinity of an exoplanet. Code description The code includes the following processes for an exospheric atom: • Collision with an UV photon • Charge-exchange with stellar wind protons • Elastic collision with another hydrogen atom • Ionization by stellar photons or wind electrons • Gravity of the star and planet, Coriolis and tidal forces Fig. 2: Slice of modeled 3D atomic hydrogen coronae around the “Hot Jupiter” HD 209458b and the corresponding Ly- transit spectrum in comparison with the HST observation. (Kislyakova et al., in preparation, 2014) After the hydrogen cloud is computed, the Ly- in-transit attenuation of the parent star can be estimated. This allows us to make conclusions about parameters in the upper atmosphere of an exoplanet, its magnetic moment and the stellar wind environment. Magnetic moment of an exoplanet defined from stellar wind density and velocity and magneto-sphericstand-off distance (Khodachenko et al., ApJ 744:70, 2012) Kepler-11 system Coronae modeling Evolutionary studies The modeling performed for five Kepler-11 “super-Earths” (Fig. 1) indicates the formation of huge hydrogen coronae around these exoplanets. The shape of the clouds is defined by the radiation pressure, charge-exchange and Coriolis force. The nonthermal ion pickup rates are estimated to be in the rangewhich is a few percent of the thermal escape rates (Kislyakova et al., A&A 562, A116, 2014) The DSMC code includes at present two species: neutral hydrogen and hydrogen ions. The code also considers a velocity-dependent radiation pressure. The method allows to model the neutral hydrogen coronae around planets, and the estimation of the ion pick-up escape rates, as well as the calculation of the corresponding Ly- attenuation of the host star if the observations are available. Activity variations of the host star allows to study the evolution of the atmospheres. Fig. 3: Illustration of the protoplanet after the nebula gas dissipation (Lammer et al., MNRAS 439, 3225, 2014) Fig. 4: Sliced of modeled atomic hydrogen coronae around a ”super-Earth” hydrogen-rich planet inside an M star HZ at 0.24 AU. Green: protons, yellow: H atoms, blue: ENAs flying away from the star, red: ENAs flying towards the star; dotted line: magnetopause/planetary obstacle. (Kislyakova et al., Astrob. 13, 1030, 2013) In combination with a hydrodynamic upper atmosphere code (Erkaev et al., Astrob. 13, 1011, 2013) and the DSCM algorithm we can estimate the thermal and nonthermal ion pick-up escape. According to Lammer et al. (MNRAS 439, 3225, 2014), Erkaev et al. (Astrob. 13, 1011, 2013), Kislyakova et al. (A&A 562, A116, 2014) the thermal escape rates are of more importance during the early XUV active stage of the host star. According to our studies, in the habitable zone of Sun-like stars only planets with masses ≤2 MEarth can rid off their primordial H-He atmospheres and evolve as rocky habitable planets. Fig. 1: Slices of modeled 3D atomic hydrogen coronae around the five Kepler-11 “super-Earths”. Blue and red dots correspond to neutral hydrogen atoms and hydrogen ions, which include stellar wind protons, respectively. The black dot in the center represents the planet. The white empty area around the planet corresponds to the XUV heated, hydrodynamically expanding thermosphere up to the height where Kn=0.1. (Kislyakova et al., A&A 562, A116, 2014)