2017MNRAS.464.2396L

The atmosphere of the exoplanet HD 209458b is undergoing sustained mass loss, believed to be caused by X-ray and extreme-ultraviolet (XUV) irradiation from its star. The majority of this flux is not directly observable due to interstellar absorption, but is required in order to correctly model the photoevaporation of the planet and photoionization of the outflow. We present a recovered high-energy spectrum for HD 209458 using a differential emission measure retrieval technique. We construct a model of the stellar corona and transition region for temperatures between 10^4.1 and 10⁸ K which is constrained jointly by ultraviolet line strengths, measured with the Cosmic Origins Spectrograph (COS) on the Hubble Space Telescope (HST) and X-ray flux measurements from XMM-Newton. The total hydrogen ionizing luminosity (λ < 912 Å) is found to be 10^28.26 erg s^–1, which is similar to the value for the mean activity level of the Sun. This luminosity is incompatible with energy-limited mass-loss rates estimated from the same COS data set, even the lower bound requires an uncomfortably high energetic efficiency of >40 per cent. However, our luminosity is compatible with early estimates of the mass-loss rate of HD 209458b, based on results from the HST Space Telescope Imaging Spectrograph. Precisely, reconstructed XUV irradiation is a key input to determining mass-loss rates and efficiencies for exoplanet atmospheres.