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Timescales for stellar evolution and star and planet formation are critical to provide constraints on theories. The accuracy of these timescales, and therefore our ability to confidently reject a given model, rely on the accuracy of the derived ages for star-forming-regions (SFRs). In this study I have developed the new techniques and adopted or updated the existing techniques necessary to derive precise age orders for a range of SFRs. Deriving precise ages for SFRs requires precise distances and extinctions. I have applied a new technique, 2 fitting (Naylor &Je ries, 2006), to derive a set of self-consistent and statistically robust distances (and mean extinctions), with associated uncertainties for 12 SFRs. I have also revised and formalised a widely used method of deriving individual extinctions, the Q-method (Johnson &Morgan, 1953). These new data show that the largest remaining uncertainty in deriving distances to SFRs is composition. Deriving ages or age orders for pre-main-sequence (pre-MS) populations using pre-MS theoretical isochrones has been shown to be unreliable at present (Naylor et al., 2002; Bonatto et al., 2004; Pinsonneault et al., 2004), largely due to model dependencies and spreads within a colour-magnitude diagram (CMD). Therefore, I have developed a technique to model the pre-MS, generating empirical isochrones, which e ectively removes spreads in pre-MS populations in a CMD. The derived distances and extinctions have been applied to the empirical isochrones, enabling the creation of an age ordered ladder in intrinsic colour and absolute magnitude. This has been calibrated using ages for fiducial sequences and nominal ages assigned to the separable groups, which are as follows: 1 Myr, NGC2244 and IC5146. 2 Myrs, NGC6530 and the ONC. 3 Myrs, Ori, Cep OB3b, NGC2264 and Ori. 4 5 Myrs, NGC2362 and IC348. 10 Myrs, NGC7160. 13 Myrs, h and Per. 20 Myrs, NGC1960. 40 Myrs, NGC2547. 2 Once assigned the nominal ages and age orders were combined with ancillary data to investigate rotation rate and disc evolution. The general trends of rotation rate distribution evolution and disc fraction changes with age confirmed existing estimates for the disc survival, and therefore star-disc interaction through disc-locking, with a timescale of 5 Myrs. However, this study also revealed some of the first evidence of local environment e ects. IC348 appears ‘out of sequence’ in both the rotation rate distribution and disc fraction. Specifically, IC348 has a larger disc fraction than expected at its nominal age and exhibits a rotation rate distribution expected from a much younger SFR (i.e. the ONC). This could be a consequence of the lower number density of O stars (none exist in IC348) and therefore a lower density of UV flux, which acts to hasten disc dissipation. Finally, a potentially important feature of stellar populations in a CMD, the R-C gap was identified. This separation in a CMD of the fully convective pre-MS and main-sequence (MS) stars with radiative cores was found to vary as a function of age. As the R-C gap is also measurable in colour it provides a distance independent age indicator. I have explained the underlying physics of the R-C gap and discussed possible applications of this phase change of the stellar interior. In addition, an overlap between the pre-MS and MS sections of the R-C gap was apparent in all SFRs where the R-C gap could be unambiguously identified. This R-C gap overlap shows that the studied SFRs must contain a spread in isochronal ages. However, the interpretation of this spread is dependent on the underlying assumptions. If one assumes stars form by a robust slowstar- formation (SSF) mechanism and isochronal ages represent the true age of a star, then these spreads can be construed as true age spreads. Alternatively, if one adopts a rapid-star-formation model (RSF), this spread can be explained as a variation in accretion histories of the constituent stars. As found by Siess et al. (1999) and Tout et al. (1999) accretion can act to accelerate pre-MS star evolution, meaning the isochronal age does not represent the true age of the star. This increases the advantages of empirical isochrones and age ordering over the derivation of individual ages for SFRs. Indeed, this R-C gap overlap could be used to ‘normalise out’ any spread in age or accretion history and therefore increase the power of derived age orders.

Stellar flares present challenges to the potential habitability of terrestrial planets orbiting M dwarf stars through inducing changes in the atmospheric composition and irradiating the planet's surface in large amounts of ultraviolet light. To examine their impact, we have coupled a general circulation model called the Met Office Unified Model with a chemical kinetics scheme to examine the response and changes of an Earth-like atmosphere to stellar flares and coronal mass ejections. I have implemented enhancements to the chemical kinetics framework to include the effects of photolysis, stellar energetic protons, and deposition. This was tested with a series of chemical networks with increasing complexity. I find that a M dwarf with stellar flares increases the amount of ozone in the atmosphere by a factor of 20 compared to a quiescent M dwarf. I find that coronal mass ejections abiotically generate significant levels of potential bio-signatures such as N₂O, and do not have a large impact on the amount of ozone. The changes in atmospheric composition cause a moderate decrease in the amount of ultraviolet light that reaches the planet's surface, suggesting that while flares are potentially harmful to life, the changes in the atmosphere due to a stellar flare act to reduce the impact of the next stellar flare.

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.