Interaction of exoplanetary and stellar winds

The close location of many known exoplanets to their host stars results in the intensive heating and ionization of the upper atmosphere by the stellar X-ray and EUV radiation. As a result, the ionized atmospheric material expands and is blown off in the form of a supersonic planetary wind. At higher altitudes, the escaping planetary plasma is either picked up by the stellar wind, or attracted by the stellar gravity. That constitutes the matter of the planetary mass loss. By this, the process of interaction between stellar and planetary wind was not properly treated so far. The innovation of the project consists in applying of a dedicated self-consistent hydrodynamic model of the interacting planetary and stellar winds for major types of exoplanets. The multi-fluid approach is implemented to describe dynamics of planetary atoms, ions, stellar protons, and other species, such as Energetic Neutral Atoms (ENAs) or heavy stellar ions. The model takes into account 1) a realistic solar-like spectrum of XUV radiation which ionizes and heats the upper atmosphere of an exoplanet; 2) basic photo-chemistry, in order to calculate a realistic composition of the interacting material flows and related energy conversion processes; 3) effects of the planetary magnetic field in 2D axisymmetric geometry. The goal of the proposed investigation is to characterize different regimes of the planetary and stellar winds interaction, as determined by mutual action of the tidal force and planet gravity, stellar wind pressure and XUV radiation. A special study will be done to understand the role of planetary magnetic field which affects both planetary and stellar winds, as well as energy dissipation due to different motion of ions, atoms and electrons in magnetized plasma. Potential observability of the modelled processes will be evaluated. The Space Research Institute of the Austrian Academy of Sciences (SRI-AAS) in Graz leads the Austrian project team, in which collaborators from the Institute of Astronomy of the University of Vienna (IA-UV) also take part. The Russian team, coordinated by the Institute of Laser Physics of Siberian Branch of Russian Academy (ILP- SB-RAS) in Novosibirsk, includes also collaborators from the Siberian Federal University (SFU) in Krasnoyarsk. All the participants have already a proven experience of succcessfull collaboration. The present cooperation project will supplement the on-going Austrian FWF NFN research program Pathways to Habitability and FWF project Characterizing Stellar and Exoplanetary Environments via Modeling of Ly Transit Observations of Hot Jupiters which cover various relevant areas of exoplanetary and stellar physics.

The close location of many known exoplanets to their host stars leads to intensive heating and ionization of their upper atmospheres by the stellar X-ray and EUV radiation. As a result, the partially ionized atmospheric material expands and forms a kind of escaping planetary wind. At higher altitudes, the planetary wind, driven by the gravitational, centrifugal, MHD, and stellar radiation pressure forces, interacts with the entire stellar wind and is either blown away, or accreted on the star. This constitutes the essence of the planetary mass loss. The escaping planetary wind, being often a supersonic one, affects the entire stellar system and results in many still unexplored processes. In accordance with its research program, the project was aimed at the investigation and characterization of different regimes of exoplanetary and stellar winds interaction, paying attention to their observational manifestations and the role of planetary magnetic field. Special focus was made on the simulation of complex dynamic environments of exoplanets, and interpretation of the real transmission spectroscopy data acquired in course of the project by partner astronomer teams. To achieve the project goals, several numerical codes were developed and upgraded. Based on the chemical code CHROME, the model of a multicomponent planetary atmosphere has been created, applicable to a wide range of exoplanets. It was used to calculate chemical reactions in the elaborated hydrodynamic and MHD models of the escaping exoplanetary atmospheres, which were extended to simulate the dynamics of their components (e.g., H, H2, H3, He, C, O, Mg, Si, N2, CO2) and corresponding ions, taking into account the whole range of plasma photo-chemistry reactions, particle collisions, and basic driving forces. The original 2D models were upgraded in the course of the project to global 3D numerical codes, which for the first time enabled a self-consistent simulation of the stellar winds and aeronomy of the expanding upper atmospheres of exoplanets in their mutual interaction. The developed models were used for the simulation of mass loss and interpretation of the in-transit spectral absorption measured in various lines for the exoplanets HD 209458b (Ly, OI, CII, SiI, MgI, MgII); WASP-12b (MgII); WASP-107b (HeI); Men c (Ly); GJ3470b (Ly, HeI), and GJ436b (Ly). Besides of that, to study the atmosphere evolution of Earth-like planets in the vicinity of active stars, the multi-component 1D models of N2- and CO2-dominated atmospheres were elaborated (Kompot Code). As an additional direction, the project developed the methods for the analysis of border regions of the transit light curves, provided by the Kepler Space Telescope, and in this way enabled the probing of dusty structures possibly present in the immediate vicinity of some exoplanets.