Characterizing stellar and exoplanetary environments via Ly-alpha modeling

Recently, spacecraft observations of so-called Energetic Neutral Atoms (ENAs) related to charge exchange have become an important remote-sensing technique in planetary science for analyzing the solar wind plasma flow around the upper atmospheric environments of Solar System bodies. ENAs are produced whenever solar or stellar wind protons interact via charge exchange with a neutral particle from a planetary atmosphere so that their signals constrain both, ion distributions and neutral gas densities. Because the upper atmospheres of hot exoplanets are exposed to the high SXR/EUV radiation of their host stars, the thermosphere-exosphere region can expand up to the Roche lobe and even beyond a magnetopause, which leads to a strong interaction between the stellar plasma flow and the expanding planetary neutral atoms. The strong radiation pressure will also deform the neutral hydrogen atoms. Furthermore, Doppler Broadening may also affect the spectrum in dense hot upper atmospheres. We apply 3D magnetohydrodynamic (MHD) and a 3D Direct Simulation Monte Carlo (DSMC) models to study the production of ENAs via stellar plasma charge exchange reactions and use their topology as a tool for finding answers for important open questions such as: i) the influence of short time variations of the stellar wind/CME plasma density and velocity to the upper atmosphere structures of hot Jupiters; ii) the interaction of dynamically outward flowing planetary neutral atoms with the ion outflow; iii) the influence of planetary obstacle location and shape, depending on intrinsic magnetic fields, induced magnetic fields, or ionopause to ENA-hydrogen cloud topology; iv) the influence of non-thermal produced hot atoms compared to the bulk atmosphere in hydrogen ENA cloud production and topology and finally v) the efficiency of non-thermal ion escape via stellar wind plasma erosion (charge exchange, photo ionization, electron impact ionization) and its role for atmospheric mass loss evolution scenarios. The latter point is very important for understanding the mass-loss evolution and the atmospheric stability of hot Jupiters in relation to the initial mass function of exoplanets. We will also investigate observational possibilities of the studied stellar-exoplanet plasma interaction processes with present and future UV (HST, WSO-UV) space observatories. Moreover, our planned stellar activity parameter study of the ENA-hydrogen cloud spectroscopic distribution and Lyman-a absorption spectra, as well as radiation pressure and Doppler Broadening studies around exoplanets are also important for precursor studies of future space missions (CHEOPS, TESS, PLATO) and possible hydrogen ENA cloud observations around exoplanets orbiting bright stars with different age mission, where the age of bright exoplanet host stars will be determined via astroseismology. By knowing the age of the star and its transiting exoplanet we are able to test magnetosphere evolution scenarios of close-in gas giants or could also use our applied technique for the reconstruction of the history of stellar/solar winds. Such observations together with the proposed modeling efforts would have a big and important impact for studies related to the evolution of planetary atmospheres and planetology in general.

Because the upper atmospheres of hot exoplanets are exposed to the high X-ray and extreme ultraviolet radiation of their host stars, the upper atmospheres can expand up to or even beyond the magnetopause, which leads to a strong interaction between the charged particle flow of the stellar wind and the expanding planetary neutral atoms. For that reason, hot hydrogen-dominated exoplanets look like comets that orbit close to the Sun, where the strong radiation pressure deforms their neutral hydrogen atmospheres. This topology and the production of hot energetic hydrogen particles was studied for the close environments of hot Jupiters by application of a three-dimensional magnetohydrodynamic code and a three- dimensional Monte Carlo particle code. A main result of the studies is that one cannot neglect induced magnetic fields that are generated around the planetary obstacles due to the flow of charged stellar particles, if one attempts to reproduce telescope observations of the extended transiting hydrogen atmospheres of hot Jupiters. These induced magnetic fields strongly influence the pressure equilibrium between the stellar wind ram pressure and the pressure of the outward flowing neutral and ionized atmosphere. By modeling this induced magnetic field it was discovered that HD 209458b most likely has an intrinsic magnetic moment that is about 10 20 percent that of Jupiter. Stronger or weaker intrinsic magnetic moments could not reproduce the Hubble space telescope observations of Lyman-a absorbing hydrogen halos during the transits. In case of the hot Jupiter HD 189733b, a different situation emerged. For this particular planet, the transit absorption can be modeled with typical stellar radiation and wind conditions and no intrinsic magnetic moment. It was also found that temporary exposure to stellar flare- or CME events yield negligible changes of the planetary absorption signal. Studies of the influence of stellar wind and activity parameters on Lyman-a absorption spectra are important precursor studies for future space missions (CHEOPS, TESS, PLATO), because it was shown that by applying magnetohydrodynamic models magnetic fields of hot gas giants and/or stellar wind parameters near exoplanets can be studied with UV-observations. Moreover, under consideration of our model results it was discovered that the ionized particles contained within the dynamically expanding and escaping upper atmospheres of both hot Jupiters prevent radio emission to escape from these exoplanets. This may be a reason why despite intensive search programs with radio telescopes so far no radio emission from exoplanets has been detected.