Influence of secondary ionisation by photoelectrons on exoplanetary atmospheres

Abstract

Planetary mass-loss is governed by several physical mechanisms such as: thermal hydrodynamic escape, photochemical escape, ionospheric outflow, interaction with the stellar wind and dynamical events such as flares. All these processes affect the model-estimated atmospheric mass loss rates. The phenomenon of hydrodynamic escape has long been postulated to have acted upon Earth, Venus and Mars in their early lives. The ongoing detection and characterisation of exoplanets offers a new window into the underlying physics. With this new insight, we expect to offer new clues into the formation of planets near and afar. Moreover, the stellar radiation energy deposited as heat depends strongly on the energy of the primary electrons following photoionisation and on the local fractional ionisation. We perform 1D spherical and 2D Cartesian HD simulations with the PLUTO code of an atomic hydrogen planetary atmosphere. We assess the impact of the shape of different XUV spectra taken from the MUSCLES survey of type K and M type stars on the mass loss rate and the impact of flare events on the planetary atmosphere. For the first time, we take into account the effect of secondary ionisation by photoelectrons self-consistently in different Exosystems. In our explored sample, we also vary the planetary masses and report a significant diminution up to 54% of the planetary mass loss rate for a planet with a mass of 0.02 Mj.