The Influence of Stellar Coronae on Planetary He 1083nm Transmission Spectra

Atmospheric escape, often driven largely by stellar irradiation, plays a major role in the development of young exoplanets. Stars tend to be quite active in their early life, so understanding how stellar variability influences the mass-loss rate is an important aspect of modelling young systems. The helium metastable triplet at 1083.3nm is an excellent line to study escape observationally. However, this line has been shown to be sensitive to features of the host star like elemental abundances and temperature distribution in the corona, and we explore this further here. Simulations with atmospheric escape models like ATES and sunbather reveal that the spectral lines originating from coronal plasma at different temperatures contribute to the partial or complete ionization of a substantial fraction of helium in hot Jupiter atmospheres, but the exact nature of this temperature-ionization relationship is not straightforward. This is relevant as it is the balance between the recombination of singly ionized helium and the ionization of metastable helium that ultimately affects the metastable helium population and hence the line strength. Therefore, modelling the coronal temperature structure in different activity states of a young star is important for fitting observations of escaping helium and building a more accurate picture of photoevaporation from young planets.