In this thesis I will present optical and near-infrared photometric and spectroscopic observations of an evolved field brown dwarf binary pair and of populations of low-mass stars in high-mass young stellar clusters and will compare them to stellar and substellar theoretical model predictions. Epsilon Indi Ba and Bb are the closest known brown dwarfs to the Earth, and, as such, make possible a concerted observational campaign to obtain a complete characterisation of two intermediate-age T dwarfs. Although some recent observations suggest substellar atmospheric and evolutionary models may be inconsistent with observations, there have been few conclusive tests to date. I will present high angular resolution optical, near-infrared, and thermal-infrared imaging and medium-resolution (up to R~5000) spectroscopy of these two T dwarfs. Using these data I have derived luminosities of log L/Lsun = -4.699 +/- 0.017 and -5.232 +/- 0.020 for Epsilon Indi Ba, Bb, respectively, and I will show that the predictions of substellar evolutionary models using luminosity and mass constraints are inconsistent with the effective temperatures and surface gravities derived from fitting atmospheric models to observed spectra. Furthermore, I will show that, even where estimates of the effective temperature, surface gravity, and luminosity are available, estimates of the mass of cool brown dwarfs can be up to a factor of two lower than the measured dynamical mass. Considering the difficulty in assigning accurate ages to any system and the mass-luminosity-age degeneracy of brown dwarfs, I would caution against the over-analysis of predicted brown dwarf masses at this time. I have also used Chandra X-ray observations to identify near-complete and relatively unbiased samples of pre-main-sequence stars in the young stellar clusters NGC 2244 and Trumpler 14. Using optical photometric and spectroscopic observations, I will characterise the apparent age spreads seen in the cluster pre-main-sequences. Mass estimates will then be derived for their stellar populations and used to construct the initial mass function for each cluster. It is found that NGC 2244 appears to have a Salpeter-like IMF for stars in the mass range 0.5--2.0 Msun if a likely age of 2 Myr is adopted, while Tr 14 may have a top-heavy IMF at a similar age. However, I will show that because the observed slopes of the cluster pre-main-sequences are not well-matched by the predicted slope in colour-magnitude space, such determinations are heavily dependent on the assumed age of the cluster and complicated due to the large spreads in isochronal ages.

This thesis describes the development, benchmarking and application of a non-LTE, co-moving frame Monte Carlo molecular line radiative transfer module for TORUS. Careful attention has been paid to the convergence, acceleration and optimisation of the code. I present the results of the application of the code to various benchmarking scenarios, including a collapsing cloud, a circumstellar disc and a very optically thick cloud of interstellar water. Benchmarking is an essential step in verifying the accuracy and efficiency of the code which is vital if it is to be used to analyse real data. In all cases, the code was able to accurately reproduce either the expected analytical solution or (in the absence of such a solution) was able to produce results commensurate with the results of other codes. In order to facilitate the motivating radiative transfer calculations of a star-forming cluster simulated using smoothed particle hydrodynamics (SPH) performed in this thesis, it was first necessary to devise and test an algorithm that efficiently maps an irregular distribution of smoothed particle hydrodynamics (SPH) particles onto a regular adaptive mesh. Whilst the algorithm was designed with this in mind it has also been used to study the effects of radiative feedback in circumstellar discs as well create a synthetic survey of a simulated galaxy. Bate et al.'s particle representation was resampled onto an adaptive mesh to enable me to use TORUS to obtain non-LTE level populations of multiple molecular species throughout the cluster and create velocity-resolved datacubes by calculating the emergent intensity using raytracing. I compared line profiles of cores traced by N2H+ (1-0) to probes of low density gas (13CO and C18O (1-0)) surrounding the cores along the line-of-sight. The relative differences of the line-centre velocities were found to be small compared to the velocity dispersion, matching recent observations. The conclusion is that one cannot reject competitive accretion as a viable theory of star formation based on observed velocity profiles.

The atmospheres and interiors of planetary and stellar systems has been studied in various forms, though the complexity of these systems currently makes full replication of their dynamics impossible. In order to make these complex systems tractable it is necessary to make significant simplifications. For many years models have concentrated on making use of the Boussinesq approximation, often with a constant density reference profile. More recently, anelastic approximations have been developed to allow for the analysis of some degree of reference state density variation. The validity of these approximations is well understood for modelling the adiabatic, inviscid, terrestrial atmosphere, however their use in modelling other regimes remains equivocal. We consider the fully ompressible, Boussinesq, anelastic, quasi-hydrostatic and pseudo-incompressible equation sets governing fluid flow within a rotating, differentially heated system. We consider both tangent plane and spherical shell geometries and conduct a normal model analysis in order to examine the validity of these approximations outside of terrestrial parameters. We find the compressibility approximations can cause spurious distortion of the normal mode solutions including misrepresentation of the frequencies, growth/decay rates, and modal structure. This in turn can have knock on effects including energy redistribution. The level of distortion is found to be dependent on mode type, reference profile and geometry and varies according to approximated equation set. Selected eigenmodes and frequencies are presented and discussed. We conclude that the most suitable approximated equation sets for use in modelling the various regions of the Jovian atmosphere depends primarily on the type of wave it is necessary to reflect most accurately; and that the scale analyses, upon which the approximated equation sets are based, provide a good indication of the regimes in which their use is appropriate.

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.

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It has "been known for some seven decades that the presence of a free surface, onto which condensation may take place, is required before the phase change from vapour to liquid will proceed, and that the smaller is the radius of curvature of the surface, the higher the degree of super saturation of the vapour must "be to induce the phase change. Such surfaces were found to be present in any normal sample of atmospheric air in the form of condensation nuclei, ranging in size and in effectiveness as centres at which condensation would take place from relatively large dust particles, effective at very small degrees of supersaturation of less than 1%t to minute clusters of vapour molecules, stabilized by the presence of a charge, requiring a supersaturation greater than 400% to be effective.

This thesis is concerned with the study of near-planetary rotation period oscillations in Saturn’s magnetic field. Similar oscillations are present in a variety of other magnetospheric phenomena, most notably kilometre-wavelength radio emissions, and together they have a substantial influence throughout the Saturnian plasma environment. However, their origin remains poorly understood, despite more than a decade of study. Surprising new discoveries have been made regarding the seasonality of these oscillatory phenomena, and their intrinsic connection to the high-latitude upper atmosphere and ionosphere of the planet, though a complete theory of their origin remains to be developed. In this thesis, three detailed studies of this phenomenon are presented, each of which uses magnetic field data recently obtained by the Cassini spacecraft. The first consists of an extensive survey of near-equatorial field data during southern summer, from which the structure of the rotating oscillations is determined and the electrical currents flowing through the equatorial plane are calculated. The second study is prompted by the recent discovery of a north-south asymmetry in the period of related radio emissions, and consists of a survey of high-latitude magnetic field data in which evidence for a corresponding asymmetry in the magnetic field rotation period is found. Finally, the third study concerns the presence of long-termdrifts between the phase of the magnetic field oscillation and the Saturn kilometric radiation (SKR) modulation, which, through construction of a simple theoretical model, is shown to arise as a consequence of ‘rotational’ modulations in the SKR rather than the hitherto assumed ‘strobe-like’ modulations. These three studies substantially further our understanding of these phenomena, and in the case of the detection of a rotational modulation in the SKR overturn three decades of previous thought. Consequently, the results presented here elucidate characteristics of these phenomena that any theoretical understanding of their origin must serve to explain, and will therefore be of central importance in further development of this topic in the future. Before presenting these three studies, an introduction to the topics of space plasma physics and planetary magnetospheres is presented in Chapter 1. This is followed in Chapter 2 with a discussion of Saturn’s magnetosphere, and the topic of near-planetary period oscillations, and in Chapter 3 by descriptions of the instruments with which data used in this thesis have been obtained.

In cosmological simulations of structure formation of the universe, the dark matter halos in which galaxies are embedded in, are triaxial in shape. Simulations which focus in on galaxies in isolation do not use triaxial dark matter halos but use spherical shaped halos instead, as triaxial equilibrium models are hard to create.;Using a new method of creating initial conditions, which is able to create triaxial equilibrium models, the effect of using a triaxial halo in simulations of satellite accretion will be studied. In previous studies with a spherical shaped dark matter halo, the initial position of the satellite is unimportant due to the symmetry of the system, however in a triaxial halo this is not the case and the initial position becomes important. A study of the parameter space of the initial position and velocity of a satellite is undertaken and the orbital decay is compared in each case.;The different mergers will also have different effects upon the halo itself as the orbital energy of the satellite is transferred to the halo. The effects of a minor merger onto the halo is investigated and compared to the scenario more commonly used when the halo is spherical.;This study is then extended to find out whether the orbital decay in a triaxial potential, which can differ significantly from that in a spherical halo, can be used to explain the observations of polar ring galaxies and gas discs in elliptical galaxies.

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