Monday, June 25, 2012

1206.5003 (Pascal Tremblin et al.)

Colliding Planetary and Stellar Winds: Charge Exchange and Transit Spectroscopy in Neutral Hydrogen    [PDF]

Pascal Tremblin, Eugene Chiang
When transiting their host stars, hot Jupiters absorb about 10% of the light in the wings of the stellar Lyman-alpha emission line. Surprisingly, the absorption occurs at wavelengths Doppler-shifted from line center by \pm100 km/s-much larger than the speeds with which partially neutral, 7000 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. 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. We observe the contact discontinuity where the two winds meet to be Kelvin-Helmholtz unstable. The Kelvin-Helmholtz instability mixes the two winds; in the mixing layer, charge exchange reactions establish, within tens of seconds, a chemical equilibrium in which the neutral fraction of hot stellar hydrogen equals the neutral fraction of cold planetary hydrogen (about 20%). In our simulations, enough hot neutral hydrogen is generated to reproduce the transit observations, and the amount of absorption converges with both spatial resolution and time. We provide physical scaling relations that describe how the absorption varies with stellar and planetary wind properties; in particular the absorption scales positively but weakly with both the stellar and planetary wind densities.
View original: http://arxiv.org/abs/1206.5003

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