Hydrodynamical interaction of stellar and planetary winds: effects of charge exchange and radiation pressure on the observed Ly$\alpha$ absorption

Abstract

Lyman $\alpha$ observations of the transiting exoplanet HD 209458b enable the study of exoplanets exospheres exposed to stellar EUV fluxes, as well as the interacting stellar wind properties. In this study we present 3D hydrodynamical models for the stellar-planetary wind interaction including radiation pressure and charge exchange, together with photoionization, recombination and collisional ionization processes. Our models explore the contribution of the radiation pressure and charge exchange on the Ly$\alpha$ absorption profile in a hydrodynamical framework, and for a single set of stellar wind parameters appropriate for HD 209458. We find that most of the absorption is produced by the material from the planet, with a secondary contribution of neutralized stellar ions by charge exchange. At the same time, the hydrodynamic shock heats up the planetary material, resulting in a broad thermal profile. Meanwhile, the radiation pressure yielded a small velocity shift of the absorbing material. While neither charge exchange nor radiation pressure provide enough neutrals at the velocity needed to explain the observations at $-100\mathrm{km}{\mathrm{s}}^{-1}$ individually, we find that the two effects combined with the broad thermal profile are able to explain the observations.

arXiv comment12 pages, 6 Figures. Accepted for publication in MNRAS

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• Irradiation-driven escape of primordial planetary atmospheres I. The ATES photoionization hydrodynamics code
• Effects of the stellar wind on the Ly-alpha transit of close-in planets
• The young HD 73583 (TOI-560) planetary system: Two 10-M${}_{\oplus }$ mini-Neptunes transiting a 500-Myr-old, bright, and active K dwarf
• Metastable Helium Absorptions with 3D Hydrodynamics and Self-Consistent Photochemistry II: WASP-107b, Stellar Wind, Radiation Pressure, and Shear Instability
• The Effects of Magnetic Fields on Observational Signatures of Atmospheric Escape in Exoplanets: Double Tail Structures
• The fundamentals of Lyman-alpha exoplanet transits
• GJ 436b and the stellar wind interaction: simulations constraints using Ly$\alpha$ and H$\alpha$ transits
• Stellar Winds Drive Strong Variations in Exoplanet Evaporative Outflows and Transit Absorption Signatures
• On the origin of the non-detection of metastable HeI in the upper atmosphere of the hot Jupiter WASP-80b
• Influence of the Sun-like magnetic cycle on exoplanetary atmospheric escape
• Star-Planet Interactions: A Computational View
• Stellar Wind Effect on the Atmospheric Escape of Hot Jupiters and their Ly-$\alpha$ and H$\alpha$ transits
• Effects of Charge Exchange on the Evaporative Wind of HD 209458b
• Stellar wind impact on early atmospheres around unmagnetized Earth-like planets
• Emerging planetary nebulae within 3D spiral patterns
• Star-planet interaction through spectral lines
• Atmospheric mass loss and stellar wind effects in young and old systems I: comparative 3D study of TOI-942 and TOI-421 systems