  When transiting their host stars, hot Jupiters absorb about 10% of the light
in the wings of the stellar Lyman-alpha emission line. The absorption occurs at
wavelengths Doppler-shifted from line center by +/- 100 km/s - larger than the
thermal speeds with which partially neutral, 10E4 K hydrogen escapes from hot
Jupiter atmospheres. It has been proposed that the absorption arises from 10E6
K hydrogen from the host stellar wind, made momentarily neutral by charge
exchange with planetary H I. The +/-100 km/s velocities would then be
attributed to the typical velocity dispersions of protons in the stellar wind -
as inferred from spacecraft measurements of the Solar wind. To test this
proposal, we perform 2D hydrodynamic simulations of colliding hot Jupiter and
stellar winds, augmented by a chemistry module to compute the amount of hot
neutral hydrogen produced by charge exchange. The Kelvin-Helmholtz instability
mixes the two winds; in the mixing layer, charge exchange reactions establish,
within tens of seconds. Enough hot neutral hydrogen is generated to reproduce
the transit observations, and the amount of absorption converges with both
spatial resolution and time. Our calculations support the idea that charge
transfer between colliding winds correctly explains Lyman-alpha transit
observations - modulo the effects of magnetic fields, which we do not model but
which may suppress mixing. Other neglected effects include, in order of
decreasing importance, rotational forces related to orbital motion, gravity,
and stellar radiation pressure; we discuss quantitatively the errors introduced
by our approximations. How hot stellar hydrogen cools when it collides with
cold planetary hydrogen is also considered; a more careful treatment of how the
mixing layer thermally equilibrates might explain the recent detection of
Balmer H-alpha absorption in transiting hot Jupiters.
