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The behaviour of convection within stellar interiors is turbulent and remains an ill- constrained factor within stellar models, despite advancements in computational mod- elling and astroseismic measurements. The impact of overshooting on the evolution of stellar structures is well known, but despite many proposals for its formulation, there is still no universal agreement to its application. This investigation attempts to better con- strain and improve upon the formulation of overshooting and related physical phenomena for 8-30M⊙ massive stars and the solar interior. Observational constraints for double eclipsing binary systems and main sequence stars derived from stellar population den- sity maps were used to fit stellar models with 0.1 ≤ α_ov ≤ 0.5. Stellar structure data was extracted from models constrained by observations, and used to construct variable over-shooting relationships to provide evolving values of α_ov during the MS. It was found that logarithmic or power-law relationships between α_ov and either L or M_co were the most suited to reproduce the MS band coverage predicted by models between 8-20M⊙. The impact of local heating below the solar convective zone boundary was implemented as ∇T modification in 1D solar models. These modifications were able to reproduce the sound- speed profile reported for helioseismic measurements, which standard solar models were found to have discrepancies to.

The focus of this work is the development and improvement of chemistry schemes in both 1D and 3D atmosphere models, applied to exoplanets. With an ever increasing number of known exoplanets, planets orbiting stars other than the Sun, the diversity in the physical and chemical nature of planets and their atmospheres is becoming more apparent. One of the prime targets, and the focus of many observational and theoretical studies, are the subclass of exoplanets termed hot Jupiters, Jovian sized planets on very short period orbits around their host star. Due to their close orbit, with orbital periods of just a few days, the atmospheres of such planets are heated to very high temperatures (~1000-2000 K) by the intense irradiation from the star. In addition, it is expected that these planets should have synchronised their rotation with their orbital period, a phenomenon called tidal-locking, that leads to a permanently illuminated dayside and a perpetually dark nightside. This combination of intense heating and tidal-locking leads to an exotic type of atmosphere that is without analogue in our own Solar system. Observational constraints suggest that some of these atmospheres may be clear whilst others may be cloudy or contain haze. Some hot Jupiters appear to be inflated with radii larger than is expected for their mass. For the warmest hot Jupiters optical absorbing species TiO and VO are expected to be present, due to the thermodynamical conditions, where they can strongly influence the thermal structure of the atmosphere, yet so far these species have remained elusive in observations. Theoretical simulations of these planets appear to provide poor matches to the observed emission flux from the nightside of the planet whilst providing a much better agreement with the observed dayside flux. These outstanding questions can be tackled in two complimentary ways. Firstly, the number of exoplanets subject to intense observational scrutiny must be increased to improve the statistical significance of observed trends. Secondly, and in tandem, the suite of available theoretical models applied to such atmospheres must be improved to allow for a more comprehensive understanding of the potential physical and chemical processes that occur in these atmospheres, as well as for better comparison of model predictions with observations. In this thesis we present the development and application of one-dimensional (1D) and three-dimensional (3D) models to the atmospheres of hot exoplanets, with a focus on improving the representation of chemistry. One of the concerns of this work is to couple the radiative transfer and chemistry calculations in a one-dimensional model to allow for a self-consistent model that includes feedback between the chemical composition and the thermal structure. We apply this model to the atmospheres of two typical hot Jupiters to quantify this effect. Implications for previous models that do not include this consistency are discussed. Another major focus is to improve the representation of chemistry in the Met Office Unified Model (UM) for exoplanet applications, a three-dimensional model with its heritage in modelling the Earth atmosphere that has recently been applied to exoplanets. We discuss the coupling of two new chemistry schemes that improve both the flexibility and capabilities of the UM applied to exoplanets. Ultimately these developments will allow for a consistent approach to calculate the 3D chemical composition of the atmosphere taking into account the effect of large scale advection, one of the processes currently hypothesised to cause the discrepancy between model predictions and observations of the nightside emission flux of many hot Jupiters.

The young moving groups are collections of nearby (<200 pc), young (5-150 Myr) pre-main sequence stars; these stars offer us one of the best opportunities to characterise stellar multiplicity, sub-stellar phenomena, disc evolution and planet formation. Here we present results from a series of multiplicity studies aimed at producing comprehensive multiplicity statistics of the young moving groups. The aim was to compare the derived statistics of the young moving groups to other populations in order to investigate whether the abundance and properties of multiple systems are environment independent. We have combined high-resolution spectroscopy, AO-imaging and direct imaging to identify and characterise multiple systems across a huge range of orbital periods (1- 10e10 day). The observational techniques also allow us to constrain the abundance of multiple systems in these populations by calculating detection limits. We found many similarities (frequency of spectroscopic binaries; frequency, mass-ratio and physical separation of visual binaries) between the young moving groups and both younger and older regions, for multiple systems with physical separations smaller than 1000 au. We did, however, identify a significant number of new wide (>1000 au) companions. We reconciled the apparent excess of wide binary systems, when compared to the field population, by arguing that the wide systems are weakly bound and most likely decaying. By comparing the multiplicity statistics in one particular moving group we showed that the dynamical evolution of non-hierarchical protostars could lead to the population of wide binaries we can observe today. Our results indicate that the majority of low-mass stars form in small groups with 3 or 4 components that undergo significant dynamical evolution. The multiplicity properties of the young nearby moving groups are statistically similar to many other populations, supporting the environment-independent formation of multiple systems.

To date more than 1500 exoplanets have been discovered. A large number of these are hot Jupiters, Jupiter-sized planets orbiting < 0.1 au from their parent stars, due to limitations in observational techniques making them easier to detect than smaller planets in wider orbits. This is also, for the same reasons, the class of exoplanets with the most observational constraints. Due to the very large interaction between these planets and their parent stars they are believed to be tidally locked, causing a large temperature contrast between the permanently hot day side and colder night side. There are still many open questions about these planets. Many are observed to have inflated radii, i.e. the observed radius is larger for a given mass than evolutionary models predict. A mechanism that can transport some of the stellar heating into the interior of the planet may be able to explain this. The presence of hazes or clouds has been inferred on some planets, but their composition and distribution remain unknown. According to chemical equilibrium models TiO and VO should be present on the day side of the hottest of these planets, but these molecules have not yet been detected. Cold traps, where these molecules condense out on the night side, have been suggested to explain this. The efficiency of the heat redistribution from the day side to the night side has been found to vary significantly between different planets; the mechanism behind this is still unknown. To begin to answer many of these questions we need models capturing the three-dimensional nature of the atmospheres of these planets. General circulation models (GCMs) do this by solving the equations of fluid dynamics for the atmosphere coupled to a radiative transfer scheme. GCMs have previously been applied to several exoplanets, but many solve simplified fluid equations (shallow water or primitive equations) or highly parametrised radiation schemes (temperature-forcing, gray or band-averaged opacities). We here present an adaptation of the Met Office Unified Model (UM), a GCM used for weather predictions and climate studies for the Earth, to hot Jupiters. The UM solves the full 3D Euler equations for the fluid, and the radiation scheme uses the two-stream approximation and correlated-k method, which are state of the art for both Earth and exoplanet GCMs. This makes it ideally suited for the study of hot Jupiters. An important part of this work is devoted to the adaptation of the radiation scheme of the UM to hot Jupiters. This includes calculation of opacities for the main absorbers in these atmospheres from state-of-the-art high temperature line lists, the calculation of k-coefficients from these opacities, and making sure all aspects of the scheme perform satisfactorily at high temperatures and pressures. We have tested approximations made in previous works such as the two-stream approximation, use of band-averaged opacities and different treatments of gaseous overlap. Uncertainties in current models, such as the lack of high temperature line broadening parameters for these atmospheres, are discussed. We couple the adapted radiation scheme to the UM dynamical core, which has been tested independently. Our first application is devoted to one of the most well-observed hot Jupiters, HD 209458b. Differences between previous modelling works and our model are discussed, and we compare results from the full coupled model with results obtained using a temperature-forcing scheme. We have also developed a tool to calculate synthetic phase curves, and emission and transmission spectra from the output of our 3D model. This enables us to directly compare our model results to observations and test the effect of various parameters and model choices on observable quantities.