2016MNRAS.458.3880T

It has been suggested that hot stellar wind gas in a bow shock around an exoplanet is sufficiently opaque to absorb stellar photons and give rise to an observable transit depth at optical and UV wavelengths. In the first part of this paper, we use the cloudy plasma simulation code to model the absorption from X-ray to radio wavelengths by 1D slabs of gas in coronal equilibrium with varying densities (10⁴-10⁸/cm3) and temperatures (2000-10⁶ K) illuminated by a solar spectrum. For slabs at coronal temperatures (10⁶ K) and densities even orders of magnitude larger than expected for the compressed stellar wind (10⁴-10⁵/cm3), we find optical depths orders of magnitude too small (>3 x 10^–7) to explain the ∼3 per cent UV transit depths seen with Hubble. Using this result and our modelling of slabs with lower temperatures (2000-10⁴K), the conclusion is that the UV transits of WASP-12b and HD 189733b are likely due to atoms originating in the planet, as the stellar wind is too highly ionized. A corollary of this result is that transport of neutral atoms from the denser planetary atmosphere outward must be a primary consideration when constructing physical models. In the second part of this paper, additional calculations using cloudy are carried out to model a slab of planetary gas in radiative and thermal equilibrium with the stellar radiation field. Promising sources of opacity from the X-ray to radio wavelengths are discussed, some of which are not yet observed.