The relative role of EUV radiation and X-rays in the heating of hydrogen-rich exoplanet atmospheres

¹ INAF – Osservatorio Astronomico di Cagliari, Strada 54, Loc. Poggio dei Pini, 09012 Capoterra (CA), Italy e-mail: ccp@ca.astro.it
² INAF – Osservatorio Astronomico di Palermo, Piazza Parlamento 1, 90134 Palermo, Italy e-mail: [ciarave;giusi;tpenz]@astropa.unipa.it

Received: 11 April 2008
Accepted: 5 January 2009

Abstract

Aims. 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.

Methods. An accurate photo-ionization model has been used to follow the primary photo-electron energy deposition throughout the atmosphere.

Results. 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 cm^-2 only X-rays can heat the gas. Extreme ultraviolet photons heat the upper atmospheric layers.

Conclusions. 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.