2009A&A...496..863C

We study the relative role of EUV and X-ray radiation in the heating of hydrogen-rich planet atmospheres with different composition and electron content. An accurate photo-ionization model has been used to follow the primary photo-electron energy deposition throughout the atmosphere. Heating rates and efficiencies have been computed, together with column density cut-offs at which photons of given energies stop their heating production inside the atmosphere. Assuming 100 eV as the energy borderline between the extreme ultraviolet spectral range and X-rays we find that when the absorbing hydrogen column density is higher than 10²⁰/cm2 only X-rays can heat the gas. Extreme ultraviolet photons heat the upper atmospheric layers. Using emission spectra from a sample of solar-type stars of different ages representative of the Sun's main sequence lifetime, we have derived the corresponding heating rates. We find that the existence of an energetic cross-over in atmospheric heating is present for all stars in the sample.