  Gas giant exoplanets orbiting at close distances to the parent star are
subjected to large radiation and stellar wind fluxes. In this paper,
hydrodynamic simulations of the planetary upper atmosphere and its interaction
with the stellar wind are carried out to understand the possible flow regimes
and how they affect the Lyman-alpha transmission spectrum. Following Tremblin
and Chiang, charge exchange reactions are included to explore the role of
energetic atoms as compared to thermal particles. In order to understand the
role of the tail as compared to the leading edge of the planetary gas, the
simulations were carried out under axisymmetry, and photoionization and stellar
wind electron impact ionization reactions were included to limit the extent of
the neutrals away from the planet. By varying the planetary gas temperature,
two regimes are found. At high temperature, a supersonic planetary wind is
found, which is turned around by the stellar wind and forms a tail behind the
planet. At lower temperatures, the planetary wind is shut off when the stellar
wind penetrates inside where the sonic point would have been. In this regime
mass is lost by viscous interaction at the boundary between planetary and
stellar wind gases. Absorption by cold hydrogen atoms is large near the
planetary surface, and decreases away from the planet as expected. The hot
hydrogen absorption is in an annulus and typically dominated by the tail, at
large impact parameter, rather than by the thin leading edge of the mixing
layer near the substellar point.
