Mixed Modes and the Asteroseismic Surface Term

We present new methodological developments regarding the treatment of the asteroseismic surface term in stars with mixed modes. Models of solar-like oscillators yield acoustic modes at different frequencies than would be seen in actual stars possessing identical interior structure, due to modelling error near the surface. This asteroseismic ``surface term'' must be corrected when mode frequencies are used to infer stellar structure. This is typically done by way of likelihood functions intended to diagnose whether or not differences between two sets of mode frequencies are consistent with a structural perturbation localised to the stellar surface. Different choices of these prescriptions modify the posterior distributions of fundamental stellar properties — such as the mass, radius, and age — inferred from individual stars using asteroseismology. These in turn induce population-level systematic biases.  Existing prescriptions developed for p-modes are also not immediately applicable to the mixed modes seen in more evolved solar-like oscillators. We examine some outstanding issues in how the surface term is currently treated in stellar modelling with asteroseismology, and how these new methods may address these shortcomings, with a particular focus on the interaction between mode mixing and these systematic effects.