Reconstructing the high energy irradiation of the evaporating hot Jupiter HD 209458b

TL;DR

This study reconstructs the high-energy XUV spectrum of exoplanet HD 209458b using a DEM technique, providing crucial data for modeling its atmospheric mass loss due to stellar irradiation.

Contribution

It introduces a novel DEM-based method to recover the star's high-energy spectrum constrained by UV and X-ray observations, improving understanding of exoplanet atmospheric evaporation.

Findings

Hydrogen ionising luminosity similar to the Sun's activity level

Energy-limited mass loss rates are incompatible with the reconstructed luminosity

Reconstructed XUV spectrum aligns with earlier mass loss estimates from HST/STIS

Abstract

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 photo-evaporation of the planet and photo-ionisation of the outflow. We present a recovered high energy spectrum for HD\,209458 using a Differential Emission Measure (DEM) retrieval technique. We construct a model of the stellar corona and transition region for temperatures between 10$^{4.1}$ and 10$^{8}$ 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 ionising luminosity ($\lambda < 912$ \AA) 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 dataset, even the lower bound requires an uncomfortably high energetic efficiency of >40%. 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 (STIS). Precisely reconstructed XUV irradiation is a key input to determining mass loss rates and efficiencies for exoplanet atmospheres.