Three-dimensional Hydrodynamical Simulation of the Exoplanet HD 209458b

Abstract

HD 209458b is a Jovian planet orbiting around a star with characteristics similar to the Sun, which has repeated transits across the stellar disk. The orbital period is of 3.5 days, with a mean distance to the star of 0.045 AU. The peculiar characteristic of this system is that the planet shows a larger apparent size when observed in Lyα (1214-1217 Å) than at optical wavelengths. This discrepancy is thought to be due to the existence of an extended upper atmosphere and/or a cometary wake. In this work we present 3D numerical simulations of this system, and study the absorption produced by the extended atmosphere. This exosphere is modeled as a wind outflowing from the planet at the escape velocity (60 km s-1), which interacts with the stellar wind. Four runs where carried out with planetary mass loss rates in the 1.5 × 10-17 to 1.5 × 10-14 M☉ yr-1 range. By comparing the Lyα absorption predicted from the models with the observations, we determine an p = (1.8 ± 0.4) × 10-16 M☉ yr-1 value for the mass loss rate from HD 209458b (a value which is of course dependent on the model assumptions, the error reflecting only the propagation of the observational uncertainties). This mass loss rate is consistent with the lower limit for p (=1010 g s-1) determined by Vidal-Madjar and coworkers.