3D hydrodynamic simulations of the planetary system GJ436: constraining the planetary wind parameters through spectroscopic observations.

GJ436b was the first detection of atmospheric escape from a warm-Neptune around an M-dwarf. The extreme absorption observed in the Ly-α line during transit indicates the existence of a very long tail of planetary neutral material trailing the planet, not being completely swept by the interaction with the stellar wind. Previous work based on particle simulations (Bourrier et al. 2016, 2015; Ehrenreich et al. 2015) have shown that the Ly-α absorption profile can be reproduce with a high planetary wind velocity (50-70 km/s) and a very low stellar wind velocity and temperature (80 km/s and 2×10³ K). As GJ436b lies in the edge of the sub-Jovian desert, the characterisation of the stellar and planetary wind parameters will help to understand the process involved in the erosion of the planetary atmosphere, and will give us a hint on the possibles scenarios that lead to the formation of such desert. In this talk we will present the unpublished results of the first 3D hydrodynamics simulations of GJ436 planetary system. These simulations take into account the stellar and planetary wind interaction and include the process of charge-exchange, radiation pressure and photoionization. With the output of the simulations we can compute synthetic Ly-α transit profiles that we compare with the observations. We will show that observed absorption profiles in Ly-α can be reproduced in a hydrodynamic scenario where the interaction of both winds forms a shock ahead of the planet. We will also show the constrained parameters obtained for the planetary wind and the influence that charge-exchange and radiation pressure have on the escaping planetary material. The new derived values are not in agreement with the ones presented in the work of Bourrier et al. (2016), showing that the observations in Ly-α alone can not be used to constrain the planetary wind parameters. In this sense, we also explore synthetic observations of the H-α line during transit.