The C/O ratio and its influence on hydrodynamic escape from Super-Earth exoplanets

Photoevaporating planetary winds have been proposed as important mechanism contributing to the evolution of the volatile inventory for close-in planets, potentially explaining the observed radius valley seen in the population of low-mass exoplanets at around R ~ 1.7r_Earth. Molecules originating in a planets’ lower atmosphere can be entrained in the escaping hydrogen wind and dissociated by energetic photons on their way to higher altitudes. This process results in outflows containing varying amounts of C and O atoms, in their neutral and ionized states. These minor species can contribute strongly to the total cooling capability in the outflow, and hence impact the mass loss rates significantly. In this work, I use a 1-D multi-species radiation hydrodynamics code in conjunction with photochemistry, in order to investigate the C/O-dependent mass loss-rates and thermal structure of outflows from those exoplanets. I will discuss impacts of the birth properties of exoplanets, parameterized by their atmospheric C/O and C/H values. Furthermore I will investigate the evolution of exo-atmospheres under the UV-rich irradiation conditions of M and K dwarves. Finally, those radius evolution scenarios will give rise to predictions of the observable transit radii of planets at a given age. Incorporating constraints of C/O measurements from JWST as well as ground-based high-resolution spectroscopy will help test those predictions and understand the role of atomic cooling in planetary evolution.