Project – Exoplanet Demographics

In recent years, a bimodal distribution has been observed in the sizes of small, close-in exoplanets, often referred to as the ‘radius gap’. Currently there are two atmospheric evolution models which can provide explanations as to how the radius gap may arise, namely XUV photoevaporation and core-powered mass-loss. Both of these models propose that whilst larger planets that are further away from their host star can maintain an extended atmosphere, those that are smaller and closer may have their atmospheres removed due to irradiation from the host star. The models differ however, as to which part of the stellar spectrum is responsible for this atmospheric mass-loss. Photoevaporation relies primarily on the extreme ultra-violet and x-ray photons (XUV), whilst core-powered mass-loss requires the entire spectrum of stellar irradiation. In this work, this fact has been utilised to present a model comparison test to determine which of the models is the dominant in exoplanet evolution. DiRAC was implemented to model ~10^6 planets undergoing one of these two proposed mass-loss mechanisms. The radius gap that is then formed is analysed and compared to that which is observed in the Gaia Kepler Survey (GKS). The radius gap should vary differently with host stellar mass and incident flux received from the host star depending on the different models. With current data, we see that the measurements of the valley slope is consistent between the models and the GKS survey (see Figure). We have concluded that a future of survey of ~5000 planets with a wide range in stellar masses will be able to use this technique to determine which of these mass-loss mechanisms is dominant in exoplanet evolution.

Project – Stellar Variability

This project is concerned with modelling the spectra of stars commonly found to be the hosts of exoplanets. The objective is to provide reference UV spectra for a range of spectral types, magnetic fields and observational angles to estimate the stellar contamination of an exoplanet spectrum due to bright active regions (faculae). UV is of particular interest because of important atmospheric tracers having their signatures in this wavelength range. The more realistic non-LTE treatment has to be adopted for this work. Furthermore, the currently available 3D MHD models of stellar interiors allow for a detailed representation of the conditions in which the stellar spectrum is formed as opposed to 1D models reflecting only the average conditions in a stellar atmosphere. The figure shows the images of intensity at 187.5 nm for two MHD simulations of different magnetizations as seen from different inclination angles (required to model the movement of an active region across the stellar disk) and calculated in LTE and non-LTE regimes. The SSD magnetization represents the non-active surface of a solar-like star, while 300G magnetization represents a facular region. The bottom row of the figure shows the ratio of middle to top rows to highlight the correlation of non-LTE effects with the convective structure of the stellar atmosphere.